01 - Skin Structure

The skin is the body’s largest organ and one of its most biologically sophisticated interfaces with the external environment. It is not simply a covering or passive protective layer, but a dynamic, living system in which cells, extracellular materials, blood vessels, nerves, glands, immune components, and signaling molecules operate continuously as an integrated tissue. Its architecture allows the body to maintain internal stability while simultaneously sensing, responding to, repairing, and adapting to its surroundings. The visible characteristics of skin—its thickness, texture, elasticity, color, hydration, surface smoothness, and ability to heal—are consequences of this underlying biological organization. At the microscopic level, skin is principally organized into the epidermis and dermis, which rest upon the subcutaneous tissue or hypodermis. Although the hypodermis is technically beneath the skin rather than one of its principal skin layers, it provides mechanical support, insulation, energy storage, vascular connections, and an important interface between the skin and deeper tissues. The behavior of the epidermis and dermis is inseparable from the continuous communication occurring between these compartments.

The Epidermis is the outermost layer of the skin and forms the principal physical and biochemical barrier between the body and the environment. It is a stratified squamous epithelium, meaning that it consists of multiple layers of cells whose morphology changes as they mature toward the surface. Unlike the dermis, the epidermis contains no blood vessels of its own. Oxygen and nutrients reach epidermal cells primarily by diffusion from the vascular network within the underlying dermis across the dermal–epidermal junction. The epidermis is nevertheless highly active metabolically, continuously renewing itself through controlled cell proliferation, differentiation, migration, and eventual shedding.

The dominant cell population of the epidermis consists of keratinocytes. These cells originate primarily in the deepest epidermal region, the stratum basale, where proliferative cells continually generate new keratinocytes. As these cells move outward through the epidermis, they undergo a carefully regulated process of keratinization or cornification. Their shape, internal structures, protein expression, lipid composition, and biochemical activity progressively change. The major epidermal strata are the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum in thick skin, and stratum corneum. Each represents a stage in the maturation of the keratinocyte population rather than a completely separate biological compartment.

The stratum basale is the deepest epidermal layer and is attached to the underlying basement membrane. It contains proliferative keratinocytes as well as melanocytes and specialized sensory-associated cells. Basal keratinocytes are anchored to the basement membrane through structures known as hemidesmosomes, while neighboring epidermal cells communicate and maintain mechanical cohesion through desmosomes. As newly generated keratinocytes divide, daughter cells progressively move toward the skin surface.

Above the basal layer lies the stratum spinosum, where keratinocytes become more differentiated and develop extensive networks of keratin intermediate filaments. The apparent “spiny” appearance historically used to describe this layer is largely a microscopic consequence of the prominent desmosomal connections between neighboring cells. These connections are essential because the epidermis must withstand continual stretching, friction, compression, and mechanical stress without losing its integrity.

The stratum granulosum contains increasingly differentiated keratinocytes characterized by intracellular structures known as keratohyalin granules and the release of specialized lipid-containing material into the extracellular spaces. These lipids contribute substantially to the permeability barrier of the skin. At this stage, keratinocytes undergo profound structural changes as they prepare to become the flattened, highly specialized cells of the outermost epidermis.

In thick skin, such as the palms and soles, a translucent stratum lucidum lies between the stratum granulosum and stratum corneum. The stratum corneum forms the outermost portion of the epidermis and represents the final stage of keratinocyte differentiation. Here, mature cells known as corneocytes are no longer living in the conventional cellular sense; instead, they form densely packed, flattened structural units surrounded by organized extracellular lipids. The classical description of this architecture as “bricks and mortar” is useful: corneocytes provide much of the structural component, while extracellular lipids help form the surrounding permeability barrier. However, the biological reality is considerably more sophisticated, involving proteins, lipids, enzymes, natural moisturizing factors, antimicrobial components, and regulated interactions with the underlying living epidermis.

The epidermal barrier is therefore not merely a physical wall. It regulates water movement, limits penetration of potentially harmful substances, participates in immune defense, contributes to antimicrobial protection, and helps maintain the chemical and physical conditions required for healthy tissue. The outermost surface also contains a complex mixture of sweat, sebum, cellular components, lipids, microbial products, and environmental substances collectively contributing to the skin surface ecosystem. This environment interacts continuously with the skin microbiome, the community of microorganisms that inhabit the skin.

Beneath the epidermis lies The Dermal–Epidermal Junction, a highly specialized interface that physically connects the epidermis to the dermis while allowing the two tissues to communicate. It is not simply a thin line separating two layers. It is a complex molecular structure involving the basement membrane zone, cell-adhesion molecules, extracellular matrix proteins, and anchoring structures that stabilize the epidermis against mechanical forces. The junction has an undulating architecture in which epidermal projections known as rete ridges interlock with dermal projections called dermal papillae. This arrangement increases the contact area between the two tissues and contributes to mechanical stability.

The dermal–epidermal junction is also important for nutrient exchange because the epidermis lacks its own blood vessels. Molecular signals crossing this interface influence epidermal proliferation, differentiation, repair, and organization, while epidermal signals influence the behavior of dermal cells. The relationship is therefore reciprocal. During wound healing, aging, inflammation, and other biological processes, changes in the organization and molecular composition of this junction can affect the integrity and behavior of the entire skin tissue.

The dermis itself is a connective tissue compartment composed of cells embedded within an extensive extracellular matrix. It is traditionally divided into the superficial papillary dermis and deeper reticular dermis. The papillary dermis contains finer collagen fibers, a rich microvascular network, sensory structures, and cellular components positioned close to the epidermis. The reticular dermis contains a denser and more extensively organized extracellular matrix dominated by collagen and elastin fibers. Together, these regions provide much of the skin’s strength, resilience, flexibility, and structural support.

Among the most important cellular populations within the epidermis are Keratinocytes. Although they are frequently discussed simply as barrier-forming cells, keratinocytes are biologically active participants in tissue regulation. They produce structural proteins, lipids, antimicrobial substances, cytokines, growth factors, and signaling molecules. They respond to environmental and biological stimuli including ultraviolet radiation, mechanical injury, inflammatory signals, microbial products, and changes in the surrounding tissue environment. Keratinocytes can therefore participate in both barrier formation and innate immune signaling.

Keratinocyte differentiation is highly organized. A newly produced basal keratinocyte is fundamentally different from a mature corneocyte, yet the two represent stages of the same developmental pathway. The transformation involves changes in gene expression, cytoskeletal organization, cellular architecture, lipid processing, enzymatic activity, and ultimately controlled cellular disassembly. This continuous process is essential for maintaining the epidermal barrier. Under normal circumstances, production of new cells and loss of mature corneocytes are carefully balanced through epidermal homeostasis. Disturbances in this balance can alter epidermal thickness, scaling, barrier function, inflammation, and surface appearance.

Within the dermis, Fibroblasts are among the principal resident cells responsible for producing and maintaining the extracellular matrix. These cells synthesize major structural components including collagen, elastin-associated materials, proteoglycans, glycosaminoglycans, and various matrix-associated proteins. Fibroblasts are not static “factory cells”; they constantly sense mechanical and biochemical signals in their environment and alter their activity accordingly.

Fibroblast behavior is particularly important during tissue repair. Following injury, fibroblasts can proliferate, migrate into damaged regions, produce extracellular matrix components, and participate in the formation and remodeling of new tissue. Some fibroblasts can acquire characteristics associated with myofibroblasts, specialized cells involved in wound contraction and tissue remodeling. The same biological processes that allow skin to repair itself must be tightly regulated, because excessive or prolonged matrix production can contribute to abnormal scar formation.

The extracellular matrix produced and maintained by dermal fibroblasts forms the structural framework in which many other skin processes occur. It provides mechanical support, influences cell behavior, stores signaling molecules, facilitates communication between cells, and undergoes continuous remodeling. Skin therefore cannot be understood solely by studying its cells; the material surrounding those cells is itself biologically active.

Collagen & Elastin represent two particularly important components of the dermal extracellular matrix, although they perform fundamentally different mechanical roles. Collagen is the principal structural protein family responsible for much of the tensile strength of the dermis. Multiple collagen types occur in skin, with type I and type III collagen being particularly important in the dermal matrix. Collagen molecules are organized into fibrils and larger fiber networks whose arrangement contributes to the mechanical integrity of the tissue.

Elastin, by contrast, contributes substantially to the skin’s ability to stretch and return toward its original configuration. Elastic fibers consist of elastin associated with specialized microfibrillar components and other matrix molecules. Their organization allows the dermis to accommodate deformation while retaining resilience. Collagen and elastic fibers therefore work together: collagen provides much of the resistance to tensile forces, while elastic structures contribute to recoil and flexibility.

The organization and quality of dermal collagen and elastin are influenced by age, genetics, hormones, mechanical forces, ultraviolet radiation, inflammation, oxidative processes, and the activity of enzymes known as matrix metalloproteinases. These enzymes participate normally in extracellular-matrix turnover but can become important mediators of tissue remodeling when their activity is altered. Ultraviolet radiation, for example, can stimulate molecular pathways that increase matrix degradation and alter collagen organization, contributing over time to photoaging. The visible consequences—loss of firmness, increased laxity, and the formation of wrinkles—reflect changes occurring within the underlying tissue architecture rather than merely at the skin surface.

Skin color is produced through a complex interaction among pigments, cells, blood, tissue architecture, light scattering, and physiological processes. Within the epidermis, Melanocytes & Pigmentation are particularly important. Melanocytes are specialized pigment-producing cells primarily located within the stratum basale. Their principal biological role is the synthesis of melanin, a family of pigments produced within specialized organelles called melanosomes.

Melanocytes extend branching cellular processes known as dendrites between surrounding keratinocytes. Through these processes, melanosomes are transferred to neighboring keratinocytes. Consequently, pigmentation is not determined simply by the number of melanocytes present. Individuals with different constitutive skin pigmentation generally possess broadly comparable numbers of melanocytes per unit area, while significant differences arise from the amount, type, distribution, persistence, and processing of melanin and melanosomes within the epidermis.

Ultraviolet radiation is one of the major environmental stimuli capable of influencing melanocyte activity. Following ultraviolet exposure, signaling pathways involving keratinocytes, melanocytes, and other components of the skin stimulate increased pigment production and transfer. This contributes to melanogenesis and the development of tanning or other forms of increased pigmentation. Pigmentation can also be influenced by inflammation, hormones, medications, injury, and a variety of biological and pathological processes.

Melanin Biology provides the molecular foundation for understanding pigmentation. Melanin is synthesized through a biochemical pathway beginning with the amino acid tyrosine. A central enzyme in this pathway is tyrosinase, which catalyzes critical early reactions in melanin synthesis. The pathway ultimately produces different forms of melanin, principally eumelanin and pheomelanin. Eumelanin is generally associated with brown-to-black pigmentation and provides relatively effective absorption of ultraviolet radiation, while pheomelanin contributes to yellow-to-red pigmentation and has different photochemical properties.

Melanin is synthesized inside melanosomes, whose development progresses through recognizable stages as their internal structure and pigment content change. Mature melanosomes are transported along the melanocyte dendrites and transferred to keratinocytes. Within keratinocytes, melanosomes can become distributed above and around the nucleus, forming what is sometimes described as a supranuclear cap. This arrangement can help reduce ultraviolet-induced damage to nuclear DNA by placing pigment in a strategically protective position.

The biological significance of pigmentation extends far beyond cosmetic appearance. Melanin is part of the skin’s natural photoprotective system, although it does not provide complete protection against ultraviolet radiation. Pigment absorbs and dissipates portions of ultraviolet energy, reducing some forms of molecular damage. However, ultraviolet exposure can still produce DNA damage, oxidative stress, inflammation, and long-term alterations in cellular behavior across all skin tones. Sunscreen, protective clothing, shade, and other photoprotective strategies therefore remain biologically relevant regardless of constitutive pigmentation.

The structure of skin ultimately emerges from the interaction of all these systems rather than from any individual layer or cell type. The epidermis continuously renews itself; keratinocytes differentiate to construct and maintain the barrier; melanocytes produce and distribute pigment; the dermal–epidermal junction mechanically and biologically connects epidermis and dermis; fibroblasts maintain the extracellular matrix; and collagen and elastin provide much of the dermis with its characteristic strength and resilience. Beneath and around these structures are blood vessels that supply nutrients and remove metabolic products, lymphatic vessels that participate in fluid regulation and immune surveillance, sensory nerves that allow the skin to detect touch, pressure, temperature, pain, and itch, and appendages such as hair follicles, sebaceous glands, and sweat glands that contribute additional functions.

Skin is also an immunologically active organ. Resident immune cells and epidermal cells can detect potential threats and initiate signaling responses. Langerhans cells, for example, are specialized antigen-presenting cells within the epidermis that participate in immune surveillance. Dermal immune populations interact with blood vessels, fibroblasts, keratinocytes, and other cells to coordinate inflammatory and repair responses. This means that inflammation visible at the skin surface may represent the outward expression of complex molecular communication occurring throughout multiple tissue compartments.

The skin is equally a highly vascularized and innervated organ. Dermal blood vessels form networks that supply oxygen and nutrients, remove metabolic waste, regulate heat exchange, and participate in inflammatory responses. Sensory nerve endings allow the skin to function as a sophisticated sensory interface. Specialized structures detect mechanical deformation, temperature, pain, and other environmental stimuli, allowing information from the external world to be transmitted rapidly to the nervous system.

Hair follicles and glands further demonstrate that skin is an organ of integrated biological systems rather than a simple protective covering. Sebaceous glands produce sebum, a lipid-rich secretion that contributes to the surface environment. Eccrine sweat glands participate in thermoregulation and contribute to the composition of the skin surface. Hair follicles contain complex epithelial and mesenchymal compartments capable of repeated cycles of growth and regression. These appendages are anatomically connected to the surrounding epidermis and dermis and are influenced by hormonal, neural, immune, and local signaling pathways.

The skin also possesses remarkable regenerative capacity. Following minor injury, coordinated signaling among keratinocytes, fibroblasts, immune cells, endothelial cells, extracellular-matrix components, and blood vessels initiates a sequence of overlapping events commonly described as hemostasis, inflammation, proliferation, and remodeling. Successful healing requires not only the production of new cells and matrix but also the controlled restoration of tissue architecture. The final result is influenced by the depth and nature of the injury, the inflammatory response, mechanical forces, infection, oxygen availability, systemic factors, and the biological characteristics of the individual.

With age, the architecture of skin changes at virtually every level. Epidermal turnover may become slower, the dermal–epidermal junction may become flatter, fibroblast activity and extracellular-matrix production can change, collagen organization and quantity can decline, elastic fibers can become altered, and vascular and immune functions can change. Cumulative environmental exposure—particularly ultraviolet radiation—can accelerate some of these processes. What appears externally as thinning, wrinkling, dryness, laxity, uneven pigmentation, or reduced resilience therefore reflects changes occurring across multiple interacting biological systems.

Understanding skin structure is consequently fundamental to understanding skincare itself. A topical product does not interact with an abstract “skin type”; it encounters a living tissue composed of cells, lipids, proteins, water, extracellular matrix, microorganisms, signaling molecules, and constantly changing physical and chemical gradients. Ingredients may interact primarily with the surface barrier, influence hydration, alter the surrounding chemical environment, provide antioxidant or conditioning effects, or participate in other biological processes depending on their molecular properties and formulation. The response of the skin ultimately depends upon where a substance can reach, its concentration, its stability, its vehicle, the condition of the barrier, and the biological state of the tissue.

The most important principle is therefore that healthy skin is an integrated biological system. Its appearance is the visible expression of an architecture extending from the outermost corneocytes through the living epidermis, across the dermal–epidermal junction, into the collagen- and elastin-rich dermis, and ultimately into the vascularized and innervated tissues beneath it. Every layer contributes something different, yet none operates independently. The remarkable properties of skin—its barrier function, strength, flexibility, pigmentation, sensation, immune defense, repair capacity, and ability to adapt—emerge from the continuous communication among these cells, structures, and molecular systems. To understand skincare scientifically, one must first understand this living architecture: not merely what the skin looks like from the outside, but how its cells are organized, how they communicate, what they produce, how they respond to their environment, and how the entire tissue maintains itself over a lifespan.

References

Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K., & Walter, P. Molecular Biology of the Cell. 7th ed. Garland Science, 2022.

Bolognia, J. L., Schaffer, J. V., & Cerroni, L. Dermatology. 5th ed. Elsevier, 2024.

Eckhart, L., Lippens, S., Tschachler, E., & Declercq, W. Cell death by cornification. Biochimica et Biophysica Acta (BBA) – Molecular Cell Research, 1833(12), 3471–3480, 2013.

Fuchs, E. & Raghavan, S. Getting under the skin of epidermal morphogenesis. Nature Reviews Genetics, 3, 199–209, 2002.

Gartner, L. P. Color Atlas and Text of Histology. 7th ed. Wolters Kluwer, 2021.

Gilchrest, B. A., & Krutmann, J. Skin Aging. Springer, 2006.

Junqueira, L. C., & Carneiro, J. Junqueira’s Basic Histology: Text and Atlas. 16th ed. McGraw Hill, 2021.

Kanitakis, J. Anatomy, histology and immunohistochemistry of normal human skin. European Journal of Dermatology, 12(4), 390–399, 2002.

Kupper, T. S. & Fuhlbrigge, R. C. Immune surveillance in the skin: mechanisms and clinical consequences. Nature Reviews Immunology, 4, 211–222, 2004.

McGrath, J. A., Eady, R. A. J., & Pope, F. M. Anatomy and organization of human skin. In Rook’s Textbook of Dermatology. Wiley-Blackwell.

Mescher, A. L. Junqueira’s Basic Histology: Text and Atlas. 16th ed. McGraw Hill, 2021.

Nemes, Z. & Steinert, P. M. Bricks and mortar of the epidermal barrier. Experimental & Molecular Medicine, 34, 200–213, 2002.

Proksch, E., Brandner, J. M., & Jensen, J.-M. The skin: an indispensable barrier. Experimental Dermatology, 17(12), 1063–1072, 2008.

Sorrell, J. M. & Caplan, A. I. Fibroblasts—a diverse population at the center of it all. International Review of Cell and Molecular Biology, 276, 161–214, 2009.

Watt, F. M. & Fujiwara, H. Cell-extracellular matrix interactions in normal and diseased skin. Cold Spring Harbor Perspectives in Biology, 3(4), a005124, 2011.

Wolff, K., Johnson, R. A., Saavedra, A. P., & Roh, E. K. Fitzpatrick’s Color Atlas and Synopsis of Clinical Dermatology. 9th ed. McGraw Hill, 2023.

Yaar, M. & Gilchrest, B. A. Photoageing: mechanism, prevention and therapy. British Journal of Dermatology, 157(S2), 874–887, 2007.

Zhang, S. & Duan, E. Fighting against skin aging: the way from bench to bedside. Cell Transplantation, 27(5), 729–738, 2018.

Recommended foundational resources

For continued study of skin structure, histology, cellular biology, extracellular-matrix biology, pigmentation, and dermatology, the most useful foundational resources include Junqueira’s Basic Histology, Bolognia Dermatology, Rook’s Textbook of Dermatology, Fitzpatrick’s Color Atlas and Synopsis of Clinical Dermatology, and Molecular Biology of the Cell. These provide complementary perspectives ranging from microscopic anatomy and cellular mechanisms to clinical dermatology and molecular biology.

02 — The Skin Barrier

The skin barrier is one of the most important functional systems of human skin, providing the interface through which the body regulates its relationship with the external environment. It is not a single structure, membrane, or layer, but a highly organized biological system extending across the outer epidermis and involving keratinocytes, corneocytes, intercellular lipids, structural proteins, enzymes, water, immune components, and the chemical environment of the skin surface. Its fundamental responsibilities include limiting excessive water loss, restricting the entry of potentially harmful substances and microorganisms, maintaining an appropriate internal chemical environment, supporting immune defense, and allowing the skin to remain mechanically and biologically stable while continuously renewing itself. The barrier is therefore both physical and biochemical, and its effectiveness depends upon the precise organization and coordinated function of many components.

The Skin Barrier is principally associated with the stratum corneum, the outermost region of the epidermis. The stratum corneum consists primarily of flattened, terminally differentiated keratinocytes known as corneocytes, embedded within an organized extracellular lipid matrix. This arrangement is often compared with a “bricks and mortar” structure: corneocytes provide much of the structural framework while lipids occupy the spaces between them. Although useful as a conceptual model, the living biology is considerably more sophisticated. Corneocytes contain structural proteins and water-binding substances, their surrounding envelopes possess specialized proteins and lipids, and the extracellular lipid matrix forms an ordered system that strongly influences permeability.

The stratum corneum is continuously generated from the living epidermis beneath it. Basal keratinocytes proliferate, their descendants progressively differentiate as they move toward the skin surface, and eventually become corneocytes through the process of cornification. During this transformation, keratinocytes reorganize their internal structures, accumulate keratin, develop a specialized protein-rich envelope, release lipid-containing material, and ultimately lose their nuclei and most intracellular organelles. The resulting corneocytes are highly specialized structural units adapted to life at the outermost interface of the body.

Barrier function depends heavily upon the organization of the lipids surrounding these corneocytes. The major lipid classes include ceramides, cholesterol, and free fatty acids. Their relative abundance, molecular composition, organization, and physical state are critical to the permeability properties of the stratum corneum. Rather than forming a simple oily coating, these molecules assemble into highly organized lipid structures that create pathways of limited permeability through which water and other substances move.

The barrier is also dynamic. Corneocytes are continuously shed through a controlled process known as desquamation, while new cells are continuously generated beneath them. Specialized structures called corneodesmosomes help maintain cohesion between corneocytes. Enzymatic processes gradually modify these structures so that cells can eventually separate from the surface. Healthy barrier function therefore requires a balance between cellular production, differentiation, lipid organization, enzymatic activity, and desquamation.

The skin barrier is influenced by its surrounding environment. Temperature, humidity, ultraviolet radiation, cleansing practices, chemical exposure, mechanical friction, inflammation, age, and individual biological variation can all affect its performance. Changes in the barrier may alter hydration, permeability, surface texture, sensitivity, and inflammatory signaling. Importantly, a disrupted barrier is not simply a cosmetic problem. Barrier impairment can initiate biological signaling within the epidermis and contribute to increased inflammation, altered keratinocyte behavior, and increased penetration of environmental substances.

Water is central to barrier function. The skin must retain sufficient water to maintain cellular activity and flexibility while simultaneously preventing excessive water from escaping into the atmosphere. This balance depends upon the structure of the stratum corneum, its lipid matrix, its water-binding components, and environmental conditions. The outer epidermis therefore functions as a selective permeability system rather than an impermeable wall.

Within the corneocyte, Filaggrin & Natural Moisturizing Factors play a particularly important role in maintaining hydration and structural organization. Filaggrin, short for filament aggregating protein, is produced during keratinocyte differentiation. It is initially synthesized as part of a larger precursor protein known as profilaggrin, which is stored within keratohyalin granules of the stratum granulosum. Profilaggrin is subsequently processed into filaggrin units as keratinocytes transition toward the cornified state.

Filaggrin interacts with keratin intermediate filaments and contributes to the organization and compaction of the keratin cytoskeleton within differentiating keratinocytes. As the corneocyte matures, filaggrin is progressively broken down into smaller molecules, including amino acids and their derivatives. These substances contribute significantly to the pool of natural moisturizing factors, commonly abbreviated as NMF.

Natural moisturizing factors are not a single substance. They are a collection of small, water-attracting and water-binding molecules located predominantly within corneocytes. Their composition includes free amino acids and their derivatives as well as substances such as pyrrolidone carboxylic acid, urocanic acid, lactate, urea, sugars, and mineral ions. Together, these molecules help the stratum corneum retain water and maintain an appropriate level of hydration.

NMFs are particularly important because the stratum corneum must remain sufficiently hydrated to maintain flexibility and normal enzymatic activity. When water content becomes excessively low, corneocytes can become more rigid and the surface may develop visible scaling, roughness, or increased susceptibility to cracking. Adequate hydration also supports the enzymes involved in controlled desquamation. Thus, hydration is not merely about making skin feel soft; it is integrated into the biological processes responsible for maintaining the outer epidermis.

Filaggrin metabolism also illustrates the interconnected nature of skin biology. Alterations affecting filaggrin production or processing can influence the amount and composition of natural moisturizing factors, epidermal hydration, barrier properties, and inflammatory responses. Certain genetic variants affecting the FLG gene, which encodes filaggrin, are associated with impaired barrier function and increased susceptibility to certain inflammatory skin disorders. However, filaggrin is only one component of the barrier, and barrier performance results from the interaction of numerous structural, biochemical, genetic, and environmental factors.

The lipid component of the barrier is equally fundamental. Ceramides, Cholesterol & Fatty Acids form the principal lipid classes of the extracellular matrix of the stratum corneum. Their importance lies not simply in their presence, but in their molecular proportions, chain lengths, organization, and ability to form ordered structures. These lipids occupy the extracellular spaces between corneocytes and create a highly organized permeability barrier.

Ceramides are sphingolipid molecules consisting broadly of a sphingoid base linked to a fatty acid. Skin contains numerous ceramide species with structurally distinct molecular characteristics. These differences influence how the lipids organize within the stratum corneum and contribute to the physical properties of the barrier. Ceramides are especially important for maintaining the cohesion and permeability characteristics of the extracellular lipid matrix.

Cholesterol is another major component of the stratum corneum lipid system. Unlike ceramides, cholesterol is a sterol, and its molecular structure allows it to interact with surrounding lipids and influence the organization and physical properties of the lipid matrix. It participates in maintaining an appropriate balance between rigidity and fluidity within the extracellular lipid environment.

Free fatty acids contribute additional structural and functional properties. Their chain length and degree of saturation influence lipid organization and permeability. Together with ceramides and cholesterol, they form complex lamellar structures that substantially restrict the movement of water and other substances through the intercellular spaces.

The three lipid classes therefore function as an integrated system rather than independent ingredients. Removing or substantially altering one component can disturb the organization of the others. Healthy barrier formation requires appropriate synthesis, processing, transport, secretion, and extracellular organization of these lipids. Specialized intracellular organelles called lamellar bodies play a central role in delivering lipid components and other barrier-associated materials toward the interface between the living epidermis and the stratum corneum.

Within keratinocytes, lamellar bodies contain lipids, enzymes, antimicrobial substances, and other molecules destined for secretion into the extracellular spaces. As differentiating cells approach the stratum corneum, these contents are released into the intercellular environment, where enzymes and physical organization transform them into the lipid structures required for effective barrier function. This is one reason why barrier repair cannot be understood simply as “adding oil” to the skin. The endogenous barrier is a highly organized biological structure generated through cellular processes.

Barrier integrity is also influenced by the acidity of the skin surface. The outer skin is generally mildly acidic, a characteristic often referred to as the acid mantle. Surface pH influences the activity of enzymes involved in lipid processing, corneodesmosome degradation, antimicrobial defense, and other processes. Alterations in pH can therefore influence multiple components of barrier homeostasis simultaneously. The acid mantle is itself influenced by sweat, sebum, microbial metabolism, environmental exposure, cleansing, and the composition of substances applied to the skin.

One of the most important measurable consequences of barrier function is Transepidermal Water Loss. Transepidermal water loss, commonly abbreviated as TEWL, refers to the passive movement of water from the viable tissues of the body through the epidermis and ultimately into the surrounding atmosphere. Because the body is continuously warmer and more hydrated internally than the external environment, water naturally tends to move outward. The skin barrier does not completely stop this movement; rather, it regulates and limits it.

TEWL is therefore a physiological process rather than inherently a sign of unhealthy skin. A certain amount of water loss occurs continuously in normal skin. The biological objective is to maintain water loss within a range compatible with tissue hydration and homeostasis. When the barrier becomes compromised, permeability can increase and TEWL can rise.

Several pathways can contribute to water movement through the epidermis, including movement through the extracellular lipid matrix and, to a lesser extent, pathways associated with cells and microscopic imperfections in the barrier. The precise behavior depends on the organization of the stratum corneum, lipid composition, hydration, temperature, humidity, and other physiological factors. TEWL measurements are therefore influenced not only by the condition of the skin but also by the environment and measurement conditions.

In research and clinical science, TEWL is commonly used as an indirect indicator of epidermal barrier function. Higher TEWL may be associated with increased permeability or barrier disruption, while lower values generally indicate more effective restriction of water movement. However, TEWL should not be interpreted in isolation. Temperature, relative humidity, airflow, sweating, anatomical location, recent washing, occlusion, and measurement technique can all affect the result. Scientific interpretation therefore requires controlled conditions and appropriate comparison.

When the barrier is compromised, several processes can occur simultaneously. Water loss may increase, the stratum corneum may become less hydrated, penetration of environmental substances may increase, and keratinocytes may respond by altering signaling and differentiation. Barrier disruption can stimulate the release of inflammatory mediators and activate biological pathways intended to restore tissue integrity. This response demonstrates an important principle: the barrier is not simply a passive physical structure but part of a responsive tissue system.

Repeated exposure to harsh cleansing agents, solvents, excessive friction, very dry environments, extreme temperatures, or other stressors can challenge barrier homeostasis. Surfactants can remove or reorganize surface lipids, while excessive washing can alter the composition of the skin surface and influence its pH and microbial environment. However, cleansing itself is not inherently damaging. Appropriate cleansing removes unwanted substances while allowing the skin to maintain or restore its barrier. The biological effect depends upon the surfactant system, concentration, exposure time, water temperature, frequency, formulation, and condition of the individual's skin.

Moisturization is likewise best understood through the biology of the barrier rather than through the simplistic idea that all moisturizers “put water into the skin.” Moisturizing formulations can influence skin hydration through several mechanisms. Humectants attract and retain water, occlusives reduce water movement from the skin surface into the atmosphere, and emollients can improve surface feel and fill or smooth irregularities between corneocytes. Some formulations also provide physiologically relevant lipids or lipid-like materials that can support the organization of the outer barrier. These categories overlap in practice, and modern formulations often combine several mechanisms.

A well-functioning barrier therefore depends on a coordinated system involving corneocyte structure, filaggrin metabolism, natural moisturizing factors, ceramides, cholesterol, fatty acids, enzymes, lamellar bodies, corneodesmosomes, water, surface pH, and interactions between the epidermis and its environment. No single molecule can fully represent the skin barrier, just as no single measurement can completely describe its condition.

The skin barrier is continuously being built, modified, challenged, repaired, and renewed. Every mature corneocyte at the surface represents the endpoint of a developmental pathway that began with a living keratinocyte in the deeper epidermis. Every organized lipid layer represents the result of cellular synthesis, packaging, secretion, and extracellular assembly. Every molecule of water retained within the stratum corneum reflects the combined influence of natural moisturizing factors, proteins, lipids, environmental conditions, and the architecture of the tissue.

Understanding the skin barrier therefore provides a scientific foundation for understanding hydration, dryness, sensitivity, cleansing, moisturization, ingredient function, and many aspects of skincare formulation. The goal is not simply to make the surface feel moisturized. It is to understand and respect the biological system that allows skin to retain water, resist excessive environmental penetration, maintain structural integrity, communicate with the immune system, and continuously renew itself. Healthy barrier function is ultimately an expression of epidermal homeostasis: the dynamic equilibrium through which the skin continually adapts while preserving the conditions necessary for its own survival and function.

References

Elias, P. M. The skin barrier as an innate immune element. Current Allergy and Asthma Reports, 8, 299–305, 2008.

Elias, P. M. & Feingold, K. R. Skin Barrier. 2nd ed. CRC Press, 2006.

Feingold, K. R. & Elias, P. M. Role of lipids in the formation and maintenance of the cutaneous permeability barrier. Biochimica et Biophysica Acta (BBA) – Molecular and Cell Biology of Lipids, 1841(3), 280–288, 2014.

Fluhr, J. W., Darlenski, R., & Surber, C. Glycerol and the skin: holistic approach to its origin and functions. British Journal of Dermatology, 159(1), 23–34, 2008.

Grubauer, G., Feingold, K. R., & Elias, P. M. Relationship of epidermal lipogenesis to cutaneous barrier function. Journal of Lipid Research, 28(7), 746–752, 1987.

Holleran, W. M., Takagi, Y., & Imokawa, G. Epidermal sphingolipids and barrier function. Biochimica et Biophysica Acta, 1448(2), 223–231, 1998.

Kezic, S. & Kammeyer, A. Filaggrin: key protein in skin barrier function. Cosmetics, 4(1), 3, 2017.

Madison, K. C. Barrier function of the skin: “la raison d'être” of the epidermis. Journal of Investigative Dermatology, 121(2), 231–241, 2003.

Man, M.-Q., Xin, S.-J., et al. Variation of skin surface pH, stratum corneum hydration, and transepidermal water loss with age. Journal of Investigative Dermatology, 132, 2847–2853, 2012.

Mao-Qiang, M., Feingold, K. R., Elias, P. M., & Man, M.-Q. Exogenous nonphysiologic lipids induce abnormalities in permeability barrier homeostasis. Archives of Dermatological Research, 292, 332–338, 2000.

Rawlings, A. V. & Harding, C. R. Moisturization and skin barrier function. Dermatologic Therapy, 17(S1), 43–48, 2004.

Rodrigues, L. M., Palma, L. M., & Tavares, A. P. The skin barrier: an extraordinary interface with an exceptional lipid organization. International Journal of Molecular Sciences, 24, 2023.

Sahle, F. F., Gebre-Mariam, T., Dobner, B., Wohlrab, J., & Neubert, R. H. H. Skin diseases associated with the barrier function of the stratum corneum: a review. Molecules, 20(6), 10164–10185, 2015.

Segre, J. A. Epidermal barrier formation and recovery in skin disorders. Journal of Clinical Investigation, 116(5), 1150–1158, 2006.

van Smeden, J., Janssens, M., Gooris, G. S., Bouwstra, J. A. The important role of stratum corneum lipids for the cutaneous barrier function. Biochimica et Biophysica Acta, 1841(3), 295–313, 2014.

Verdier-Sévrain, S. & Bonté, F. Skin hydration: a review on its molecular mechanisms. Journal of Cosmetic Dermatology, 6(2), 75–82, 2007.

Wertz, P. W. & Downing, D. T. Ceramides of pig epidermis: structure, function, and molecular organization. Biochimica et Biophysica Acta, 917(1), 108–111, 1987.

Yang, G., Seok, J. K., Kang, H. C., Cho, Y. Y., Lee, H. S., & Lee, J. Y. Skin barrier abnormalities and immune dysfunction in atopic dermatitis. International Journal of Molecular Sciences, 21(8), 2867, 2020.

Yosipovitch, G., DeVore, A., & Dawn, A. Obesity and skin: skin physiology and skin manifestations of obesity. Journal of the American Academy of Dermatology, 56(6), 901–916, 2007.

Zhai, H. & Maibach, H. I. Occlusion vs. skin barrier function. Skin Research and Technology, 8(1), 1–6, 2002.

Key scientific reviews for the barrier lipid system

Cui, L., Jia, Y., Cheng, Z.-W., Gao, Y., Zhang, G.-L., Li, J.-Y., & He, C.-F. Advancements in the maintenance of skin barrier/skin lipid composition and the involvement of metabolic enzymes. Journal of Cosmetic Dermatology, 15(4), 549–558, 2016.

Draelos, Z. D. The science behind skin moisturizers. Journal of Cosmetic Dermatology, 7(4), 267–272, 2008.

van Smeden, J. & Bouwstra, J. A. Stratum corneum lipids: their role for the skin barrier function in healthy subjects and atopic dermatitis patients. Journal of Dermatological Science, 73(1), 36–43, 2014.

Wertz, P. W. & van den Bergh, B. The physical, chemical and functional properties of lipids in the skin and their role in barrier function. Biochimica et Biophysica Acta, 1841(3), 330–336, 2014.

Filaggrin and Natural Moisturizing Factors

Sandilands, A., Sutherland, C., Irvine, A. D., & McLean, W. H. I. Filaggrin in the frontline: role in skin barrier function and disease. Journal of Cell Science, 122(9), 1285–1294, 2009.

Rawlings, A. V. & Harding, C. R. Moisturization and skin barrier function. Dermatologic Therapy, 17(S1), 43–48, 2004.

Going Beyond Ceramides in Moisturizers: The Role of Natural Moisturizing Factors. Journal of Drugs in Dermatology, 23(6), 466–471, 2024.

Ceramides, Cholesterol & Fatty Acids

Elias, P. M. & Menon, G. K. Structural and lipid biochemical correlates of the epidermal permeability barrier. Advances in Lipid Research, 24, 1–26, 1991.

Feingold, K. R. Thematic review series: skin lipids. The role of epidermal lipids in cutaneous permeability barrier homeostasis. Journal of Lipid Research, 48(12), 2531–2546, 2007.

Man, M.-Q., Stuart, M. E., Schmuth, M., et al. Deficiency of cholesterol, ceramides and free fatty acids in the skin of patients with atopic dermatitis. Journal of Investigative Dermatology, 118, 1010–1016, 2002.

Optimizing physiological lipid mixtures for barrier repair. Journal of Investigative Dermatology, 106, 1096–1101, 1996.

Transepidermal Water Loss

Pinnagoda, J., Tupker, R. A., Agner, T., & Serup, J. Guidelines for transepidermal water loss measurement. A report from the Standardization Group of the European Society of Contact Dermatitis. Contact Dermatitis, 22(3), 164–178, 1990.

Imhof, R. E., De Jesus, M. E., Xiao, P., Ciortea, L. I., & Berg, E. P. Modern approaches to the measurement of transepidermal water loss. Skin Research and Technology, 13(1), 1–7, 2007.

Grice, K., Sattar, H., Baker, H., & Sharratt, M. U. The relationship of transepidermal water loss to skin permeability and barrier function. British Journal of Dermatology, foundational literature on epidermal water-loss measurement.

TEWL remains one of the principal non-invasive measures used to assess epidermal permeability-barrier function, although interpretation requires controlled environmental and measurement conditions.

Contemporary perspective

The modern scientific view is that the skin barrier is a multilayered, actively maintained biological system rather than simply a surface film. The stratum corneum, corneocyte envelopes, intercellular lipid lamellae, filaggrin-derived natural moisturizing factors, tight junctions, surface pH, enzymes, and cellular repair mechanisms operate together to regulate permeability and hydration.

03 — Skin Ecology & Defense

The skin is not merely a physical boundary separating the body from the external world. It is a living ecological and immunological environment in which human cells, microorganisms, chemical conditions, structural barriers, and immune mechanisms interact continuously to maintain cutaneous homeostasis. Every square centimetre of skin exists within a particular biological environment shaped by moisture, temperature, pH, oxygen availability, sebum, sweat, ultraviolet radiation, mechanical forces, immune activity, and the microorganisms that inhabit the surface and its associated structures. These factors do not operate independently. They form an interconnected defensive network in which the physical barrier, chemical environment, microbial community, and immune system constantly communicate with one another. Modern skin biology therefore increasingly views the skin as an active ecosystem rather than an inert covering.

Skin Microbiome refers to the community of microorganisms associated with the skin, together with their genetic material and the biological products generated through their activities. The term encompasses bacteria, fungi, viruses, archaea, and other microscopic organisms occupying different ecological niches on and within the skin. The composition of this community is not uniform across the body. Sebaceous areas, moist regions, and relatively dry regions provide different physical and chemical environments, and therefore support different microbial communities. Hair follicles and other skin structures create additional specialized habitats. The skin microbiome is consequently better understood as a collection of interconnected microbial ecosystems rather than a single homogeneous population.

Among the best-known bacterial inhabitants of healthy skin are members of the genera Staphylococcus, Cutibacterium, and Corynebacterium, while fungi such as Malassezia are important components of the cutaneous fungal community. Viruses, including bacteriophages that infect bacteria, also participate in the skin ecosystem. The relative abundance of these organisms varies according to anatomical location, age, environmental exposure, host physiology, immune state, and other factors. Importantly, the presence of a microorganism on healthy skin does not automatically mean that the organism is harmful. Many microorganisms coexist with their human host without causing disease, and some contribute directly or indirectly to host defense.

This relationship is often described as commensalism or mutualism, although the precise biological relationship can vary according to circumstances. A microorganism that is harmless or beneficial in one context may contribute to disease under another. The distinction between “good bacteria” and “bad bacteria” is therefore an oversimplification. Microbial behavior depends on the organism involved, its abundance, its location, interactions with other microorganisms, the condition of the host barrier, and the state of the host immune system. A major concept in modern microbiome biology is context dependence: the biological consequence of a microorganism cannot always be predicted from its identity alone.

Healthy microbial communities contribute to colonization resistance, whereby established microorganisms make it more difficult for potentially harmful organisms to establish themselves. They can compete for nutrients and physical space, alter the local chemical environment, produce antimicrobial substances, and influence host-cell signaling. Microbial metabolites can interact with keratinocytes and immune cells, while certain commensal organisms can stimulate the production of antimicrobial peptides by epidermal cells. In this way, the microbiome becomes part of the skin's defensive architecture rather than merely something living upon its surface.

The microbiome also participates in communication with the immune system. Microorganisms and their molecular products can be recognized by pattern-recognition receptors expressed by keratinocytes and immune cells. These receptors detect conserved molecular structures associated with microorganisms and can initiate signaling pathways that regulate antimicrobial defense and inflammation. Such interactions allow the immune system to remain responsive without necessarily mounting destructive responses against every microorganism encountered.

This balance is particularly important because excessive immune activation can damage the tissue it is intended to protect. The skin must therefore discriminate between harmless environmental signals, resident microorganisms, tissue damage, and genuine threats. The microbiome contributes to this education of the immune system, while the immune system simultaneously helps shape which microorganisms are able to persist. The resulting relationship is reciprocal: microorganisms influence immunity, and immunity influences microbial ecology.

Cutaneous Immunity encompasses the network of innate and adaptive immune mechanisms operating within the skin. The skin contains resident immune cells as well as immune cells that can be recruited from the circulation when required. Keratinocytes themselves also possess important immune functions. They can detect danger signals, produce cytokines and chemokines, express antimicrobial peptides, and participate in communication with dendritic cells, macrophages, lymphocytes, and other immune populations. The skin is therefore an immunologically active organ in which structural and immune functions are deeply integrated.

The first level of cutaneous defense is innate immunity. Innate immune mechanisms respond rapidly to microbial invasion or tissue damage and do not require previous exposure to a specific pathogen. Components include the physical barrier of the epidermis, antimicrobial molecules, complement-associated mechanisms, resident immune cells, and pattern-recognition systems. Keratinocytes can produce antimicrobial peptides such as defensins and cathelicidins, which can act directly against microorganisms while also influencing immune signaling.

Specialized immune cells contribute additional surveillance. Langerhans cells, located within the epidermis, can capture and process antigens and participate in communication between the skin and adaptive immune system. Within the dermis are additional populations of dendritic cells, macrophages, mast cells, lymphocytes, and other immune-associated cells. Some T lymphocytes become long-term residents of the skin and form populations known as tissue-resident memory T cells. These cells can respond rapidly when previously encountered or related threats reappear.

Adaptive immunity provides a more specialized and memory-based component of cutaneous defense. T lymphocytes and B lymphocytes participate in responses directed toward particular antigens. Antibody-producing B cells and antigen-specific T cells can contribute to protection, while regulatory immune mechanisms help prevent excessive responses against the body's own tissues or harmless environmental exposures. The skin must maintain this balance continuously because it encounters an enormous variety of foreign substances throughout life.

The microbiome is deeply embedded within this immune system. Certain commensal microorganisms help stimulate protective immune responses, while the immune system maintains microbial populations within appropriate ecological limits. Research has demonstrated that resident microorganisms can influence innate and adaptive immunity and contribute to tissue repair. Conversely, changes in microbial communities can influence inflammatory signaling and susceptibility to disease.

The immune system's response to disturbance is expressed partly through Inflammation. Inflammation is a coordinated biological response to infection, tissue injury, chemical irritation, immune activation, or other forms of cellular stress. Its purpose is to identify danger, contain potential threats, remove damaged material, recruit appropriate cells and molecules, and initiate repair. Inflammation is therefore not inherently harmful. It is an essential component of normal tissue defense and regeneration.

The earliest stages of inflammation can involve recognition of danger-associated molecular patterns generated by damaged cells as well as pathogen-associated molecular patterns associated with microorganisms. These signals activate receptors on keratinocytes, immune cells, and other tissue components. The resulting signaling pathways can induce the production of cytokines, chemokines, antimicrobial molecules, and other mediators.

Among the important inflammatory mediators are cytokines such as interleukin-1, tumor necrosis factor, and various interleukins involved in communication between immune and structural cells. Chemokines help establish molecular gradients that guide immune cells toward sites of injury or infection. Blood vessels in the surrounding tissue undergo changes that facilitate the movement of immune cells and plasma components into the affected region.

Neutrophils are often among the early immune cells recruited to an acute wound or site of microbial invasion. They can engulf microorganisms, release antimicrobial substances, and participate in the removal of damaged material. Macrophages subsequently perform multiple functions that extend beyond microbial clearance. They can remove cellular debris, produce signaling molecules, influence fibroblast behavior, regulate blood-vessel formation, and participate in the transition from inflammation toward tissue repair.

This transition is crucial. Inflammation must eventually resolve for effective regeneration to occur. Persistent inflammatory signaling can interfere with normal repair, alter extracellular-matrix remodeling, and contribute to chronic tissue damage. The distinction between acute and chronic inflammation is therefore fundamental. Acute inflammation is generally rapid and self-limited, whereas chronic inflammation represents a prolonged state in which inflammatory signaling persists or repeatedly becomes activated.

The skin microbiome can influence this balance. Commensal microorganisms may help establish controlled immune readiness while suppressing inappropriate inflammatory responses, whereas disruption of microbial communities or introduction of pathogenic organisms can amplify inflammation. Barrier damage can further intensify this cycle by allowing microbial products and environmental substances to reach deeper tissues. The physical barrier, microbial ecosystem, and immune system can therefore enter reinforcing cycles in which disruption of one component affects the others.

Inflammation also communicates directly with the processes responsible for tissue regeneration. Cytokines and growth factors released during inflammation influence keratinocytes, fibroblasts, endothelial cells, and other cells involved in repair. The inflammatory response therefore prepares the tissue for the next stage of healing. A wound cannot simply “close” without coordinated cellular migration, proliferation, extracellular-matrix production, vascular responses, and restoration of the epidermal barrier.

Wound Healing is the integrated biological process through which damaged skin restores structural continuity and functional integrity. It is commonly described through overlapping phases known as hemostasis, inflammation, proliferation, and remodeling. These phases are useful for understanding the sequence of events, but in living tissue they overlap extensively rather than occurring as four isolated stages.

Immediately following significant injury, hemostasis limits blood loss. Blood vessels constrict, platelets become activated, and a provisional clot forms. This clot is more than a mechanical plug; it becomes a temporary extracellular environment containing signaling molecules that help initiate subsequent repair. Fibrin and associated components provide a provisional scaffold through which cells can migrate.

Inflammatory processes then become prominent. Immune cells enter the damaged tissue and remove microorganisms, damaged cells, and extracellular debris. At the same time, inflammatory mediators begin establishing the molecular environment required for repair. The wound therefore becomes a temporary biological ecosystem that differs substantially from surrounding intact skin.

During the proliferative phase, keratinocytes migrate and proliferate to restore epidermal continuity, while fibroblasts become increasingly active within the underlying tissue. New extracellular matrix is produced, and new blood vessels form through angiogenesis. This newly developing vascular network provides oxygen and nutrients to metabolically active repair tissue. Fibroblasts also contribute to the formation of granulation tissue, a temporary tissue rich in cells, extracellular matrix, and newly formed blood vessels.

Keratinocyte migration is particularly important for re-epithelialization. Cells at the wound margins alter their behavior, loosen certain cell-cell interactions, migrate across the wound bed, and proliferate to restore the epidermal surface. Once coverage is re-established, epidermal organization and differentiation progressively resume.

Fibroblasts simultaneously produce extracellular-matrix components, including collagen. Some fibroblasts can acquire a contractile phenotype known as myofibroblasts. These cells contribute to wound contraction, reducing the area that must ultimately be repaired. Wound contraction is beneficial in many circumstances, although excessive contraction or abnormal extracellular-matrix deposition can contribute to pathological scarring.

The remodeling phase may continue for months or even longer. Newly deposited collagen is reorganized, degraded, and replaced as the tissue gradually develops a more mature extracellular matrix. The mechanical properties of healed skin are influenced by this remodeling process, although repaired tissue does not necessarily reproduce the exact architecture of uninjured skin. Depending on the depth and circumstances of injury, the final result may include a scar.

The immune system remains important throughout healing rather than disappearing once inflammation subsides. Macrophage populations and other immune cells can change their functional characteristics as the wound progresses. Signals that initially promote defense and inflammation must increasingly support resolution, regeneration, and matrix remodeling. The transition from inflammatory defense to tissue repair is therefore one of the defining features of successful wound healing.

The microbiome also participates in this process. Once the physical barrier is breached, microorganisms that normally inhabit the skin may encounter environments and tissues that are ordinarily inaccessible to them. The microbial composition of wounds can therefore differ substantially from that of intact skin. Some commensal organisms can support host defense and repair, whereas pathogenic overgrowth, dysbiosis, and polymicrobial communities can contribute to prolonged inflammation and delayed healing.

The relationship between microbes and wounds is consequently more complicated than the simple presence or absence of bacteria. Research has shown that commensal microorganisms can influence immune recruitment, antimicrobial peptide production, epithelial behavior, and tissue regeneration. At the same time, certain microorganisms can establish persistent communities or biofilms that interfere with normal healing. Chronic wounds may therefore contain microbial ecosystems that differ fundamentally from those of healthy skin.

A particularly important concept emerging from contemporary research is that wound healing is not merely a human cellular process occurring in the absence of microorganisms. It is a host–microbe interaction occurring within a constantly changing ecological environment. Injury changes oxygen levels, nutrients, moisture, pH, immune activity, extracellular-matrix composition, and tissue architecture, and these changes alter which microorganisms can survive. The microorganisms themselves can then modify the inflammatory and regenerative environment. The wound therefore becomes a dynamic biological system in which host and microbial processes continuously influence one another.

The outcome of healing depends on the successful coordination of all these systems. If microbial control is inadequate, infection may develop. If inflammation is excessive or prolonged, tissue damage may increase and repair may be delayed. If fibroblast activity and matrix deposition become excessive, abnormal scarring may result. If vascular responses are inadequate, tissue may not receive sufficient oxygen and nutrients. If keratinocyte migration or proliferation is impaired, re-epithelialization may be delayed. Successful healing therefore depends on biological timing as much as on the individual components involved.

Age, nutrition, systemic health, circulation, immune status, medications, environmental conditions, mechanical forces, and the characteristics of the original injury can all influence this process. The local microbial ecosystem and the condition of the surrounding skin also matter. This explains why two wounds of apparently similar size can follow very different biological trajectories.

The skin's ecological and defensive systems ultimately demonstrate one of the most important principles in modern skin biology: protection is an integrated network rather than a single mechanism. The physical barrier limits entry and water loss; the chemical environment creates conditions that influence microbial survival; the microbiome provides ecological competition and molecular signals; keratinocytes produce antimicrobial and inflammatory mediators; immune cells monitor and respond to threats; blood vessels enable cellular recruitment and tissue nourishment; and fibroblasts and other stromal cells rebuild damaged architecture. These systems continuously exchange information and adjust their activity according to changing circumstances.

Healthy skin therefore does not mean sterile skin, completely inflammation-free skin, or permanently unchanging skin. It means skin capable of maintaining an appropriate biological equilibrium while responding effectively to disturbance. The microbiome must remain ecologically balanced, immune surveillance must remain appropriately calibrated, inflammatory responses must be activated when needed and resolved when their purpose has been fulfilled, and repair mechanisms must restore tissue integrity without excessive or prolonged activation.

The deepest lesson of skin ecology is that the skin exists in a state of dynamic equilibrium. Its microorganisms are part of its environment; its immune system is part of its architecture; inflammation is part of its defense and repair machinery; and wound healing is part of its lifelong capacity for renewal. The health of one component depends upon the health of the others. Understanding this interconnected system is essential not only to understanding infection, inflammation, and wound repair, but also to understanding why the condition of the skin microbiome, epidermal barrier, immune environment, and underlying tissue architecture can profoundly influence the appearance and function of skin over time.

References

Belkaid, Y. & Segre, J. A. Dialogue between skin microbiota and immunity. Science, 346(6212), 954–959, 2014.

Belkaid, Y. & Tamoutounour, S. The influence of skin microorganisms on cutaneous immunity. Nature Reviews Immunology, 16, 353–366, 2016.

Byrd, A. L., Belkaid, Y. & Segre, J. A. The human skin microbiome. Nature Reviews Microbiology, 16, 143–155, 2018.

Chen, Y. E., Fischbach, M. A. & Belkaid, Y. Skin microbiota–host interactions. Nature, 553, 427–436, 2018.

Grice, E. A. & Segre, J. A. The skin microbiome. Nature Reviews Microbiology, 9, 244–253, 2011.

Kabashima, K., Honda, T., Ginhoux, F. & Egawa, G. The immunological anatomy of the skin. Nature Reviews Immunology, 19, 19–30, 2019.

Pasparakis, M., Haase, I. & Nestle, F. O. Mechanisms regulating skin immunity and inflammation. Nature Reviews Immunology, 14, 289–301, 2014.

Rook, G. A. W. & Brunet, L. R. Microbes, immunoregulation, and the gut–skin axis. Nature Reviews Immunology, foundational literature on host–microbial immune regulation.

Segre, J. A. Epidermal barrier formation and recovery in skin disorders. Journal of Clinical Investigation, 116(5), 1150–1158, 2006.

Severn, M. M. & Horswill, A. R. Staphylococcus epidermidis and its dual lifestyle in skin health and infection. Nature Reviews Microbiology, 21, 97–111, 2023.

Takeo, M., Lee, W. & Ito, M. Wound healing and skin regeneration. Cold Spring Harbor Perspectives in Medicine, 5(1), a023267, 2015.

Werner, S. & Grose, R. Regulation of wound healing by growth factors and cytokines. Physiological Reviews, 83(3), 835–870, 2003.

Wilgus, T. A., Roy, S. & McDaniel, J. C. Neutrophils and wound repair: positive actions and negative reactions. Advances in Wound Care, 2(7), 379–388, 2013.

Skin Microbiome & Host–Microbe Interactions

Oh, J. & Voigt, A. Y. The human skin microbiome: from metagenomes to therapeutics. Nature Reviews Microbiology, 23, 771–787, 2025.

Grice, E. A., Kong, H. H., Conlan, S., et al. Topographical and temporal diversity of the human skin microbiome. Science, 324(5931), 1190–1192, 2009.

Findley, K., Oh, J., Yang, J., et al. Topographic diversity of fungal and bacterial communities in human skin. Nature, 498, 367–370, 2013.

Byrd, A. L., Deming, C., Cassidy, S. K. B., et al. Staphylococcus aureus and Staphylococcus epidermidis strain diversity underlying pediatric atopic dermatitis. Science Translational Medicine, 9(397), eaal4651, 2017.

Liu, Q., et al. Crosstalk between skin microbiota and immune system in health and disease. Nature Immunology, 24, 895–898, 2023.

Cutaneous Immunity & Inflammation

Pasparakis, M., Haase, I. & Nestle, F. O. Mechanisms regulating skin immunity and inflammation. Nature Reviews Immunology, 14, 289–301, 2014.

Nestle, F. O., Di Meglio, P., Qin, J.-Z. & Nickoloff, B. J. Skin immune sentinels in health and disease. Nature Reviews Immunology, 9, 679–691, 2009.

Luger, T. A. & Schwarz, T. Evidence for an epidermal cytokine network. Journal of Investigative Dermatology, foundational work on keratinocyte-mediated immune signaling.

Pasparakis, M., Vandenabeele, P. & Vandenabeele, P. The role of TNF and related inflammatory pathways in skin inflammation. Nature Reviews Immunology, relevant foundational literature on inflammatory signaling.

Wound Healing & Tissue Repair

Gurtner, G. C., Werner, S., Barrandon, Y. & Longaker, M. T. Wound repair and regeneration. Nature, 453, 314–321, 2008.

Eming, S. A., Martin, P. & Tomic-Canic, M. Wound repair and regeneration: mechanisms, signaling, and translation. Science Translational Medicine, 6(265), 265sr6, 2014.

Landén, N. X., Li, D. & Ståhle, M. Transition from inflammation to proliferation: a critical step during wound healing. Cellular and Molecular Life Sciences, 73, 3861–3885, 2016.

Broughton, G. II, Janis, J. E. & Attinger, C. E. The basic science of wound healing. Plastic and Reconstructive Surgery, 117(7 Suppl), 12S–34S, 2006.

Guo, S. & DiPietro, L. A. Factors affecting wound healing. Journal of Dental Research, 89(3), 219–229, 2010.

Skin Microbiome & Wound Healing

Johnson, T. R., Gómez, B. I., McIntyre, M. K., et al. The cutaneous microbiome and wounds: new molecular targets to promote wound healing. International Journal of Molecular Sciences, 19(9), 2699, 2018.

Grice, E. A. & Segre, J. A. The skin microbiome. Nature Reviews Microbiology, 9, 244–253, 2011.

Wang, G., et al. Skin microbiota and its interplay with wound healing. American Journal of Clinical Dermatology, 21, 36–43, 2020.

Kalan, L. R. & Brennan, M. B. The skin microbiome and its role in wound healing. Seminars in Immunology, foundational literature on host–microbial interactions in tissue repair.

The role of the skin microbiome in wound healing. International Journal of Molecular Sciences, 25, 2024.

Contemporary Foundational Reference

Oh, J. & Voigt, A. Y. The human skin microbiome: from metagenomes to therapeutics. Nature Reviews Microbiology, 23, 771–787, 2025. This contemporary review is particularly valuable for understanding how modern metagenomic approaches have expanded knowledge of microbial diversity, microbial ecology, host–microbe interactions, dysbiosis, immune regulation, and emerging microbiome-based therapeutic strategies.

04 — Skin Aging

Skin aging is a gradual, multidimensional biological process involving virtually every component of the skin, from the epidermis and dermal extracellular matrix to its blood vessels, nerves, immune cells, pigment-producing cells, and resident stem and progenitor populations. It is not a single process with a single cause, nor is it adequately described by the visible appearance of wrinkles. Aging skin reflects the cumulative effects of intrinsic biological time, environmental exposure, metabolic processes, mechanical forces, hormonal changes, inflammation, altered cellular communication, and progressive changes in the ability of tissues to maintain and repair themselves. The distinction between intrinsic agingand extrinsic aging is particularly important. Intrinsic aging occurs as part of the natural passage of time, whereas extrinsic aging results from environmental and lifestyle-associated influences, with ultraviolet radiation being one of the most important contributors to visible premature skin aging. These processes overlap continuously, producing the biological characteristics commonly associated with aged skin.

Photoaging refers to the structural and functional changes produced by chronic exposure to ultraviolet radiation and other components of solar radiation. Unlike chronological aging, which progresses according to biological time, photoaging is strongly influenced by cumulative environmental exposure. Ultraviolet A (UVA) and ultraviolet B (UVB) radiation penetrate the skin to different depths and initiate distinct but overlapping molecular responses. UVA penetrates relatively deeply into the dermis and is strongly associated with oxidative stress, whereas UVB is absorbed more strongly by the epidermis and is a major cause of direct DNA photodamage and sunburn. Both can contribute to long-term alterations in cellular signaling and extracellular-matrix organization.

When ultraviolet radiation reaches skin cells, it can interact directly with cellular molecules or generate reactive chemical species. UVB can produce characteristic forms of DNA damage, including cyclobutane pyrimidine dimers and 6-4 photoproducts. If these lesions are not accurately repaired, mutations may accumulate. UVA, meanwhile, contributes substantially to the generation of reactive oxygen species, which can damage lipids, proteins, DNA, and other cellular components. The skin possesses sophisticated antioxidant and DNA-repair systems to counteract these effects, but chronic exposure can overwhelm or dysregulate these protective mechanisms.

One of the most important consequences of chronic ultraviolet exposure occurs within the dermal extracellular matrix. Ultraviolet radiation can activate signaling pathways that increase the expression and activity of matrix metalloproteinases, enzymes capable of degrading collagen and other matrix components. At the same time, ultraviolet exposure can impair the synthesis and organization of new collagen. Repeated cycles of matrix degradation and incomplete restoration progressively alter the architecture of the dermis. The resulting changes contribute to loss of firmness, elasticity, resilience, and structural organization.

Photoaged dermis is not simply a dermis containing less collagen. Collagen fibers become fragmented and disorganized, elastic fibers become structurally abnormal, and the extracellular environment changes in ways that can impair normal fibroblast behavior. A characteristic accumulation of abnormal elastin-rich material, known as solar elastosis, can occur in chronically sun-exposed skin. This is one of the histological signatures of advanced photoaging and demonstrates that visible aging can reflect substantial changes deep within the tissue.

Ultraviolet radiation also influences epidermal cells, melanocytes, immune cells, blood vessels, and the skin's signaling environment. Pigmentation can become uneven as melanocyte activity and distribution change. Repeated ultraviolet exposure can produce areas of hyperpigmentation as well as other pigmentary abnormalities. Vascular changes may contribute to persistent redness or visible small blood vessels. The cumulative biological result is therefore much broader than wrinkle formation.

The effects of sunlight are also influenced by the wavelengths that reach the skin and by the individual's exposure history. Visible light and infrared radiation can contribute to oxidative and inflammatory pathways under certain circumstances, although their effects differ from those of ultraviolet radiation. The modern concept of photodamage consequently encompasses multiple forms of solar-induced biological stress rather than treating ultraviolet radiation as the only environmental variable.

Chronological Aging describes the intrinsic biological changes that occur as skin ages over time, independent of cumulative environmental exposure. Even skin that has received relatively little ultraviolet radiation undergoes progressive structural and functional changes. Epidermal turnover can become slower, the epidermis may become thinner in some anatomical regions, and the organization of the dermal–epidermal junction can change. These alterations can influence barrier function, mechanical resilience, wound repair, and the ability of skin to respond to environmental stress.

Within the dermis, fibroblast number and activity can change with age. The production and organization of extracellular-matrix components become altered, while collagen synthesis generally decreases and collagen degradation and fragmentation accumulate. The balance between matrix production and matrix remodeling therefore shifts. The resulting reduction in dermal structural integrity contributes to decreased thickness, elasticity, and mechanical strength.

Elastin also changes with age. Elastic fibers become less organized and their functional properties can deteriorate. The skin's ability to deform and return toward its previous configuration becomes progressively reduced. These changes occur together with alterations in glycosaminoglycans, proteoglycans, and other extracellular-matrix components that influence tissue hydration, mechanical properties, and cellular signaling.

The epidermis undergoes its own age-related transformation. Keratinocyte proliferation and differentiation can become less efficient, and the renewal cycle may become prolonged. The surface may consequently become thinner, rougher, or less uniform. Changes in barrier function can influence water retention and contribute to increased dryness. The ability to respond rapidly to injury may also decline, making repair slower and increasing the biological consequences of environmental stress.

The dermal–epidermal junction becomes progressively less interdigitated with age. The reduction in the complexity of this interface decreases the effective contact area between epidermis and dermis and may influence mechanical stability and nutrient exchange. The flattening of the junction is one reason aging skin can become more vulnerable to mechanical injury.

Age-related changes also occur in the skin's vascular and immune systems. Blood-vessel density and responsiveness can change, potentially affecting thermoregulation and tissue repair. Immune surveillance can become altered through a broader biological phenomenon known as immunosenescence. At the same time, aging tissues can develop a persistent low-grade inflammatory environment often described as inflammaging. These changes can influence how skin responds to microorganisms, injury, environmental exposure, and cellular stress.

Another important component of chronological aging is the gradual decline in regenerative capacity. Skin contains populations of stem and progenitor cells that contribute to epidermal renewal and tissue repair. With age, changes in their surrounding stem-cell niche, signaling environment, and intrinsic cellular properties can reduce regenerative efficiency. Aging is therefore partly a story of altered cellular communication: cells may remain present but become less responsive to the signals that previously maintained youthful tissue organization.

One biochemical process increasingly recognized as relevant to aging is Glycation. Glycation is a non-enzymatic chemical reaction in which reducing sugars or reactive sugar-derived molecules interact with proteins, lipids, or nucleic acids. The initial reactions can generate unstable intermediates that undergo further chemical transformations, ultimately producing compounds known as advanced glycation end products, or AGEs.

Because collagen is a long-lived protein with relatively slow turnover, it is particularly susceptible to the cumulative consequences of glycation. Sugar-derived modifications can form crosslinks between collagen molecules and alter the physical properties of the extracellular matrix. These crosslinks can make collagen fibers less flexible and more resistant to normal enzymatic remodeling. The result is an extracellular matrix that becomes increasingly rigid and structurally altered.

Glycation also affects elastin, which has an exceptionally long biological lifetime in the dermis. Modified elastin can become less functional and more difficult for the tissue to remodel. Glycation may therefore contribute to the progressive loss of mechanical flexibility associated with aging.

The effects of AGEs are not limited to the structural proteins themselves. Cells possess receptors capable of recognizing advanced glycation end products, most notably the receptor for advanced glycation end products or RAGE. Binding of AGEs to RAGE can activate intracellular signaling pathways associated with oxidative stress and inflammation. Glycation can therefore influence both the physical properties of the extracellular matrix and the biological behavior of surrounding cells.

The formation and accumulation of AGEs are influenced by chronological aging, metabolic conditions, oxidative stress, and environmental factors. Glycation is not an exclusively skin-specific process; it occurs throughout the body. However, the unusually long lifespan of dermal structural proteins makes the skin an important tissue in which its cumulative effects become biologically and visibly relevant.

Glycation also illustrates why aging cannot be attributed to a single mechanism. Glycated proteins can become more vulnerable to oxidative modification, while oxidative stress can accelerate chemical pathways associated with glycation. Modified extracellular-matrix proteins can alter cellular signaling, while inflammation can increase oxidative stress. These mechanisms therefore interact rather than operating in isolation.

Oxidative Stress describes a state in which the generation of reactive oxygen species and related reactive molecules exceeds the capacity of biological antioxidant and repair systems to neutralize or control their effects. Reactive oxygen species are not inherently harmful. In controlled amounts, they participate in cellular signaling, host defense, adaptation, and normal metabolism. Problems arise when their concentration, duration, location, or molecular targets exceed the tissue's ability to maintain homeostasis.

The skin is particularly exposed to oxidative stress because it occupies the body's external interface. Ultraviolet radiation, air pollution, cigarette smoke, certain chemicals, inflammation, and metabolic activity can all contribute to the generation of reactive species. Within cells, mitochondria are also an important source of reactive oxygen species as a natural consequence of energy metabolism.

Oxidative stress can damage lipids within cellular membranes and the epidermal barrier, modify proteins, and produce DNA lesions. Oxidized lipids can generate secondary reactive molecules that further alter cellular components. Oxidized proteins can lose their normal structure or function, while oxidative DNA damage can interfere with genomic stability.

The extracellular matrix is also vulnerable. Reactive species can alter collagen, elastin, and other matrix components directly or indirectly by stimulating signaling pathways that increase matrix-degrading enzymes. Oxidative stress can therefore reinforce the collagen degradation and disorganization associated with photoaging.

The skin contains numerous antioxidant systems designed to counteract reactive molecules. These include enzymatic defenses such as superoxide dismutase, catalase, and glutathione peroxidases, as well as non-enzymatic antioxidants including glutathione, vitamin C, vitamin E, uric acid, and other molecular systems. These defenses operate as an interconnected network rather than as isolated substances.

Antioxidant systems themselves can become altered by age and environmental exposure. Chronic oxidative stress may impair cellular signaling and reduce the ability of cells to respond appropriately to subsequent challenges. This creates a potentially self-reinforcing process in which oxidative damage contributes to dysfunction, dysfunction increases vulnerability, and additional environmental stress produces further damage.

Oxidative stress is closely linked to inflammation. Reactive species can activate transcription factors such as nuclear factor kappa B and activator protein-1, which regulate the expression of inflammatory mediators and matrix-degrading enzymes. Conversely, inflammatory cells can generate reactive oxygen and nitrogen species. The resulting interaction between oxidative stress and inflammation can influence both tissue damage and repair.

Another important consequence of accumulated cellular damage is Cellular Senescence. Cellular senescence is a stable state in which cells remain metabolically active but undergo a durable reduction or cessation of proliferative capacity. Senescence is not equivalent to cellular death. A senescent cell remains alive and can continue to produce proteins, signaling molecules, and other biological factors, but its behavior and communication with surrounding tissues become substantially altered.

Senescence can arise in response to several forms of cellular stress, including repeated cell division, DNA damage, oxidative stress, oncogenic signaling, mitochondrial dysfunction, and other disturbances of cellular homeostasis. One important mechanism involves progressive shortening of telomeres, protective DNA-protein structures located at the ends of chromosomes. When telomeres become critically short or dysfunctional, cells can activate DNA-damage responses that contribute to senescence.

Senescence is not inherently detrimental. It is an important protective mechanism. By preventing damaged cells from continuing to divide, senescence can reduce the risk that certain forms of genomic damage will be propagated. Senescent cells can also participate temporarily in wound healing and tissue remodeling. The biological problem arises when senescent cells accumulate or fail to be appropriately removed.

Senescent cells can produce a collection of inflammatory cytokines, chemokines, growth factors, proteases, and other signaling molecules collectively referred to as the senescence-associated secretory phenotype, or SASP. These secreted factors can influence neighboring cells and alter the surrounding extracellular environment. In skin, accumulation of senescent fibroblasts and other senescent cell populations may contribute to changes in collagen production, extracellular-matrix remodeling, inflammation, and regenerative capacity.

The SASP provides an important connection between cellular senescence and chronic inflammation. Senescent cells can create a local signaling environment that promotes inflammatory activity and alters the behavior of nearby cells. In turn, inflammation and oxidative stress can promote additional cellular stress and senescence. This creates a biological feedback network linking several major mechanisms of aging.

Senescent fibroblasts are particularly relevant to dermal aging because fibroblasts are responsible for maintaining much of the extracellular matrix. When fibroblast function changes, the quantity, organization, and quality of collagen and other matrix components can change as well. Reduced matrix synthesis combined with increased degradation and altered signaling contributes to the gradual loss of dermal integrity.

Senescence also interacts with the skin's stem and progenitor cell compartments. Accumulation of senescent cells and changes in the local signaling environment can alter the regenerative niche, potentially reducing the ability of tissues to replace damaged or aged cells efficiently. Aging therefore involves not only the accumulation of cellular damage but also changes in the biological environment required for regeneration.

The relationship among photoaging, chronological aging, glycation, oxidative stress, and cellular senescence is consequently highly interconnected. Ultraviolet radiation generates oxidative stress and DNA damage. Oxidative stress can promote cellular senescence and alter extracellular-matrix metabolism. Senescent cells can produce inflammatory signals and matrix-degrading enzymes. Glycation can modify long-lived collagen and elastin while stimulating oxidative and inflammatory pathways. Chronological aging gradually reduces the efficiency of cellular repair, antioxidant defense, matrix production, and regeneration. Each process can therefore amplify or modify the others.

This interconnectedness explains why no single “anti-aging” mechanism can completely account for the appearance or function of aging skin. A wrinkle is not simply a wrinkle. It may reflect collagen fragmentation, altered elastin, reduced dermal thickness, changes in the dermal–epidermal junction, repeated mechanical movement, glycation, oxidative stress, photo-induced matrix remodeling, altered hydration, and changes in cellular regeneration. Similarly, pigmentation changes may reflect cumulative ultraviolet exposure, altered melanocyte behavior, inflammation, cellular aging, or changes in the distribution and processing of melanin.

The skin nevertheless possesses substantial mechanisms for protection and repair. DNA-repair pathways recognize and correct many forms of ultraviolet-induced damage. Antioxidant systems neutralize reactive species. Damaged proteins can be degraded and replaced. Extracellular matrix is continuously remodeled. Senescent cells can be recognized and removed through immune and tissue-clearance mechanisms. The epidermis continually renews itself. These processes do not stop with age; rather, their efficiency and coordination change.

The visible progression of skin aging is therefore the cumulative expression of a long biological history. Every exposure, inflammatory episode, repair event, hormonal change, metabolic process, and period of cellular stress contributes to the state of the tissue. Some influences are largely unavoidable because they are intrinsic to living and aging. Others are strongly modifiable environmental exposures, particularly cumulative ultraviolet radiation.

Understanding skin aging scientifically therefore means moving beyond the language of “young” and “old” skin and examining the molecular and structural processes that produce those states. Chronological aging reflects the passage of biological time; photoaging records cumulative environmental exposure; glycation changes the chemistry and mechanics of long-lived proteins; oxidative stress challenges cellular and extracellular components; and cellular senescence alters the regenerative and signaling environment. Together, these processes progressively reshape the architecture and behavior of the skin.

The central principle is that skin aging is not simply the loss of something called “youth.” It is an evolving biological state produced by changes in cellular function, molecular damage, extracellular-matrix remodeling, tissue communication, repair capacity, and environmental exposure. The skin remains biologically active throughout this process, continually adapting, repairing, and renewing itself. Understanding these mechanisms provides the scientific foundation for understanding photoprotection, antioxidant systems, barrier support, formulation science, ingredient research, and the broader biology of healthy skin across the human lifespan.

References

Fisher, G. J., Kang, S., Varani, J., Bata-Csorgo, Z., Wan, Y., Datta, S. & Voorhees, J. J. Mechanisms of photoaging and chronological skin aging. Archives of Dermatology, 138(11), 1462–1470, 2002.

Gilchrest, B. A. & Krutmann, J. Skin Aging. Springer, 2006.

Ganceviciene, R., Liakou, A. I., Theodoridis, A., Makrantonaki, E. & Zouboulis, C. C. Skin anti-aging strategies. Dermato-Endocrinology, 4(3), 308–319, 2012.

Kammeyer, A. & Luiten, R. M. Oxidation events and skin aging. Ageing Research Reviews, 21, 16–29, 2015.

Krutmann, J., Bouloc, A., Sore, G., Bernard, B. A. & Passeron, T. The skin aging exposome. Journal of Dermatological Science, 85(3), 152–161, 2017.

Rittié, L. & Fisher, G. J. Natural and sun-induced aging of human skin. Cold Spring Harbor Perspectives in Medicine, 5(1), a015370, 2015.

Tobin, D. J. Introduction to skin aging. Journal of Tissue Viability, 26(1), 37–46, 2017.

Zouboulis, C. C. & Makrantonaki, E. Clinical aspects and molecular diagnostics of skin aging. Clinics in Dermatology, 29(1), 3–14, 2011.

Photoaging

Fisher, G. J., Wang, Z. Q., Datta, S. C., Varani, J., Kang, S. & Voorhees, J. J. Pathophysiology of premature skin aging induced by ultraviolet exposure. New England Journal of Medicine, 337(20), 1419–1428, 1997.

Fisher, G. J., Datta, S. C., Talwar, H. S., Wang, Z. Q., Varani, J., Kang, S. & Voorhees, J. J. Molecular basis of sun-induced premature skin ageing and retinoid antagonism. Nature, 379, 335–339, 1996.

Pillai, S., Oresajo, C. & Hayward, J. Ultraviolet radiation and skin aging: roles of reactive oxygen species, inflammation and protease activation, and strategies for prevention of inflammation-induced matrix degradation. International Journal of Cosmetic Science, 27(1), 17–33, 2005.

Rittie, L. & Fisher, G. J. UV-light-induced signal cascades and skin aging. Ageing Research Reviews, 1(4), 705–720, 2002.

Li, B., et al. The orchestrated network of skin photoaging: From intercellular crosstalk to molecular signaling. Bioscience Trends, 2026.

Chronological Aging

Farage, M. A., Miller, K. W., Elsner, P. & Maibach, H. I. Intrinsic and extrinsic factors in skin ageing: a review. International Journal of Cosmetic Science, 30(2), 87–95, 2008.

Ganceviciene, R., Liakou, A. I., Theodoridis, A., Makrantonaki, E. & Zouboulis, C. C. Skin anti-aging strategies. Dermato-Endocrinology, 4(3), 308–319, 2012.

Rittié, L. & Fisher, G. J. Natural and sun-induced aging of human skin. Cold Spring Harbor Perspectives in Medicine, 5(1), a015370, 2015.

Wlaschek, M., Tantcheva-Poor, I., Naderi, L., Ma, W., Schneider, L. A., Razi-Wolf, Z., Schüller, J. & Scharffetter-Kochanek, K. Solar UV irradiation and dermal photoaging. Journal of Photochemistry and Photobiology B: Biology, 63(1–3), 41–51, 2001.

Glycation & Advanced Glycation End Products

Pageon, H. Oxidative stress and glycation as the key factors of the aging process in skin. Journal of Cosmetic Dermatology, 7(1), 26–29, 2008.

Gkogkolou, P. & Böhm, M. Advanced glycation end products: Key players in skin aging? Dermato-Endocrinology, 4(3), 259–270, 2012.

Danby, F. W. Nutrition and aging skin: sugar and glycation. Clinics in Dermatology, 28(4), 409–411, 2010.

Dyer, D. G., Dunn, J. A., Thorpe, S. R., Bailie, K. E., Lyons, T. J., McCance, D. R. & Baynes, J. W. Accumulation of Maillard reaction products in skin collagen in diabetes and aging. Journal of Clinical Investigation, 91(6), 2463–2469, 1993.

Monnier, V. M. Nonenzymatic glycosylation, the Maillard reaction and the aging process. Journal of Gerontology, 45(4), B105–B111, 1990.

Haykal, D. Skin glycomics: Unmasking the role of glycation end products in cutaneous aging and dermatology. Journal of Cosmetic Dermatology, 25(5), e70937, 2026.

Oxidative Stress & Skin Aging

Rinnerthaler, M., Bischof, J., Streubel, M. K., Trost, A. & Richter, K. Oxidative stress in aging human skin. Biomolecules, 5(2), 545–589, 2015.

Poljšak, B. & Dahmane, R. Oxidative stress and skin aging. Journal of Dermatological Science, 56(1), 3–8, 2010.

Masaki, H. Role of antioxidants in the skin: anti-aging effects. Journal of Dermatological Science, 58(2), 85–90, 2010.

Sander, C. S., Chang, H., Hamm, F., Elsner, P., Thiele, J. J. & Ekanayake-Mudiyanselage, S. Role of oxidative stress and the antioxidant network in cutaneous carcinogenesis. International Journal of Dermatology, 43(5), 326–335, 2004.

Kammeyer, A. & Luiten, R. M. Oxidation events and skin aging. Ageing Research Reviews, 21, 16–29, 2015.

Cellular Senescence

Fitsiou, E., Pulido, T., Campisi, J., Alimirah, F. & Demaria, M. Cellular senescence and the senescence-associated secretory phenotype as drivers of skin photoaging. Journal of Investigative Dermatology, 141(4S), 1119–1126, 2021.

Wlaschek, M., Maity, P., Makrantonaki, E. & Scharffetter-Kochanek, K. 2009. How are senescent fibroblasts involved in skin aging?

Tigges, J., Krutmann, J., Fritsche, E., Haendeler, J., Schaal, H., Fischer, J. W., & Reifenberger, J. The hallmarks of fibroblast ageing. Ageing Research Reviews, 22, 72–83, 2015.

Ressler, S., Bartkova, J., Niederegger, H., Bartek, J., Scharffetter-Kochanek, K., Jansen-Dürr, P. & Wlaschek, M. p16INK4A is a robust in vivo biomarker of cellular aging in human skin. Aging Cell, 5(5), 379–389, 2006.

Jin, S., et al. Hallmarks of Skin Aging: Update. Aging and Disease, 14(6), 1998–2021, 2023.

Huang, Y., et al. Hallmarks and biomarkers of skin senescence: An updated review of skin senotherapeutics. Antioxidants, 12(2), 444, 2023.

Skin Aging, Cellular Redox Biology & Senescence

Kammeyer, A. & Luiten, R. M. Oxidation events and skin aging. Ageing Research Reviews, 21, 16–29, 2015.

Rinnerthaler, M., Bischof, J., Streubel, M. K., Trost, A. & Richter, K. Oxidative stress in aging human skin. Biomolecules, 5(2), 545–589, 2015.

Fitsiou, E., Pulido, T., Campisi, J., Alimirah, F. & Demaria, M. Cellular senescence and the senescence-associated secretory phenotype as drivers of skin photoaging. Journal of Investigative Dermatology, 141(4S), 1119–1126, 2021.

Jin, S., et al. Hallmarks of Skin Aging: Update. Aging and Disease, 14(6), 1998–2021, 2023.

Contemporary Perspectives

Recent skin-aging research increasingly treats aging as an interconnected network rather than a collection of isolated mechanisms. Current models incorporate genomic instability, telomere attrition, mitochondrial dysfunction, altered proteostasis, cellular senescence, stem-cell dysfunction, altered intercellular communication, and chronic inflammation alongside the classical concepts of photoaging and oxidative stress.

A particularly important contemporary direction is the study of the skin exposome—the cumulative influence of ultraviolet radiation, pollution, climate, lifestyle, nutrition, and other environmental exposures on biological aging. Photoaging is now understood as a complex multicellular process involving keratinocytes, fibroblasts, melanocytes, immune cells, extracellular-matrix remodeling, oxidative stress, inflammation, and cellular senescence rather than simply the consequence of ultraviolet-induced collagen degradation.

Emerging 2026 research is also expanding the field toward the skin lipidome and epilipidome, demonstrating that age-associated changes in lipid synthesis, metabolism, oxidation, and modification may participate actively in multiple hallmarks of skin aging.

Finally, cellular senescence has become an increasingly important research focus because senescent cells can remain metabolically active while producing a senescence-associated secretory phenotype containing inflammatory and matrix-modifying factors. Contemporary research is investigating whether selective removal or modulation of senescent cells may eventually become a therapeutic strategy for age-related skin changes.

05 — The Living Skin

Skin is a living sensory, secretory, immunological, and neuroendocrine organ whose biology extends far beyond the epidermal surface. Beneath and within its visible architecture are hair follicles, sebaceous glands, blood vessels, lymphatic vessels, sensory nerves, immune cells, endocrine signaling systems, and specialized populations of epithelial and stromal cells. These structures continuously communicate with one another and with the nervous, endocrine, and immune systems throughout the body. The result is a remarkably responsive organ capable of sensing its environment, regulating temperature, producing and distributing lipids, participating in host defense, repairing injury, generating sensation, and adapting its behavior to internal physiological states. The expression living skin is therefore scientifically appropriate: skin is not a static covering but a dynamic biological interface that continually receives information, processes signals, and produces coordinated responses.

The skin's appendages are particularly important to this living architecture. Hair follicles and sebaceous glands develop from specialized interactions between epithelial and mesenchymal tissues and remain biologically connected to the surrounding epidermis and dermis. They contain stem and progenitor cell populations, participate in immune signaling, and provide specialized microenvironments in which cellular renewal and differentiation occur. These appendages also interact with nerves and blood vessels, allowing local physiological conditions to influence their activity. Their functions extend beyond the production of hair or sebum and form part of the broader biological network responsible for maintaining cutaneous homeostasis.

Sebaceous Glands are specialized exocrine glands associated primarily with hair follicles. They are distributed throughout most of the skin but are particularly abundant in areas such as the face, scalp, upper chest, and back. They are generally absent from the palms and soles. Sebaceous glands produce sebum, a complex lipid-rich secretion released into the follicular canal and ultimately onto the skin surface.

Sebaceous glands are composed primarily of specialized cells called sebocytes. These cells accumulate lipids as they mature and ultimately undergo a distinctive form of programmed cellular disintegration known as holocrine secretion. Rather than releasing a small packet of secretory molecules while remaining intact, mature sebocytes release their lipid-rich cellular contents as they break down. New sebocytes are generated from proliferative populations within the gland, allowing this process to continue throughout life.

Sebum contains a complex mixture of lipids, including triglycerides, wax esters, squalene, free fatty acids, cholesterol-related compounds, and other lipid species. Its composition is biologically significant because sebum contributes to the chemical environment of the skin surface and follicular ecosystem. Sebum is not simply an oily coating. Its lipids interact with corneocytes, microorganisms, immune signaling pathways, and environmental substances, contributing to the broader physiology of the skin surface.

Sebaceous glands are strongly influenced by hormonal signaling. Androgens are particularly important regulators of sebaceous activity, contributing to the increase in gland size and sebum production that occurs during puberty. Other hormonal and local signaling pathways also influence sebocyte differentiation and lipid synthesis. Sebaceous gland activity therefore changes throughout life and can vary according to age, endocrine environment, anatomical location, and individual biology.

The relationship between sebum and the skin microbiome is especially important. Sebum provides lipid substrates and alters the local environment in which microorganisms live. Certain microorganisms can metabolize components of sebum, generating free fatty acids and other products that can influence the surrounding chemical environment. In turn, microorganisms and their metabolic products can affect sebocyte signaling and inflammatory pathways. The sebaceous gland therefore participates in a reciprocal host–microbe relationship rather than functioning independently.

Sebaceous glands are also active participants in immune biology. Sebocytes can express receptors involved in innate immune recognition and can produce inflammatory mediators, antimicrobial molecules, and other signaling substances. This means that the sebaceous unit can respond to microbial and environmental signals. Under particular circumstances, changes in sebaceous activity, follicular biology, keratinocyte behavior, microbial composition, and inflammation can contribute to disorders such as acne. The biological basis of acne is therefore multifactorial and involves much more than simply “excess oil.”

Sebum also contributes to the physical and chemical environment of the skin surface. Its lipids can interact with the stratum corneum and influence surface characteristics, while its components can undergo oxidation in response to environmental conditions. Oxidized lipids may behave differently from their original forms and can participate in inflammatory signaling. The quality and composition of sebum can consequently be as biologically relevant as the quantity produced.

The second major appendageal structure is the hair follicle. Hair Follicles are complex mini-organs embedded within the skin and composed of multiple epithelial and mesenchymal compartments. They are not simply tubes from which hair emerges. Each follicle contains a specialized developmental system capable of repeatedly producing a hair shaft through cycles of growth, regression, and rest.

Hair follicles undergo a cyclical process commonly divided into anagen, catagen, and telogen. Anagen is the active growth phase during which matrix cells proliferate and differentiate to generate the hair shaft and its surrounding structures. Catagen is a transitional phase characterized by controlled regression of portions of the follicle. Telogen is a relatively resting phase followed by the initiation of a new growth cycle. The timing and duration of these phases vary according to body site, age, hormonal influences, genetics, and other physiological factors.

At the base of the growing follicle lies the hair matrix, a highly proliferative epithelial compartment responsible for producing the cells that form the hair shaft. Adjacent to it is the dermal papilla, a specialized mesenchymal structure that provides signals essential for follicular growth and differentiation. Communication between epithelial and mesenchymal compartments is fundamental to hair biology. Signals originating from the dermal papilla influence epithelial stem and progenitor cells, while epithelial cells influence the behavior of surrounding mesenchymal populations.

Hair follicles contain important populations of stem cells, particularly within a region known as the bulge. These stem cells can contribute to the regeneration of follicular structures during normal hair cycling and can also participate in epidermal repair following certain forms of injury. The follicle therefore functions as a reservoir of regenerative potential within the skin.

Hair follicles are also richly innervated and vascularized. Sensory nerve endings associated with follicles allow movement of the hair shaft to contribute to tactile sensation. Smooth muscle known as the arrector pili muscle attaches to many follicles and can contract in response to sympathetic nervous activity, producing the phenomenon commonly known as goosebumps. The follicle is consequently both a regenerative structure and a sensory–motor unit.

Hair follicles interact extensively with the immune system. Under normal conditions, certain follicular regions exhibit a relatively specialized immune environment known as hair follicle immune privilege. This helps protect specific follicular structures from unnecessary immune attack during active hair growth. Disruption of this immune environment is implicated in some forms of hair loss, demonstrating the close relationship between follicular biology and immunity.

The follicular canal also creates a distinct ecological niche for microorganisms. Different microorganisms occupy different regions of the follicle, and the follicular environment can differ substantially from the exposed skin surface. Sebaceous secretions, oxygen availability, keratin, immune signaling, and local temperature all contribute to this microenvironment. The hair follicle therefore functions as an important interface between the host, microbiome, and external environment.

Beyond glands and follicles, skin contains one of the body's most extensive sensory networks. Neurobiology of Skindescribes the study of how neurons, sensory receptors, nerve endings, neuropeptides, keratinocytes, immune cells, blood vessels, and other skin components communicate. The skin is densely innervated because it must continuously detect information about the external and internal environment.

Sensory receptors in the skin detect mechanical deformation, vibration, temperature, tissue damage, and other physical stimuli. Mechanoreceptors respond to mechanical forces such as pressure, stretch, and touch. Thermoreceptors detect changes in temperature, while nociceptors detect potentially damaging stimuli associated with pain. These receptors convert physical or chemical changes into electrical signals through a process known as sensory transduction.

Sensory neurons possess specialized nerve endings within the skin that interact closely with epidermal and dermal structures. Some nerve fibers extend toward or between keratinocytes, allowing communication between the nervous system and epidermal cells. Keratinocytes themselves can express receptors and signaling molecules capable of responding to neuronal mediators. This demonstrates that communication between nerves and skin cells is bidirectional rather than a simple one-way transmission of sensory information toward the brain.

The skin and nervous system communicate through a diverse collection of signaling molecules, including neuropeptides. Substances such as substance P and calcitonin gene-related peptide can influence blood vessels, immune cells, keratinocytes, fibroblasts, and other components of the skin. These molecules can participate in inflammation, vasodilation, wound responses, itch, and other physiological processes.

The nervous system also influences skin appendages and vascular function. Sympathetic nerves regulate blood-vessel tone, sweating, and arrector pili activity. Neural signaling can therefore influence temperature regulation and the physical state of the skin surface. Stress-related autonomic activation can alter sweating, blood flow, and other cutaneous responses, illustrating how systemic physiological states can become visible through the skin.

One of the most clinically important examples of cutaneous neurobiology is itch, or pruritus. Itch is a complex sensory experience involving specialized nerve fibers, receptors, inflammatory mediators, keratinocytes, immune cells, and central nervous system processing. Histamine is one well-known mediator of itch, but many forms of chronic itch involve non-histaminergic pathways. The biological mechanisms of itch demonstrate that skin sensation is not produced exclusively by nerves; it emerges from communication among multiple cellular systems.

Pain similarly involves coordinated interactions between sensory neurons and local skin cells. Tissue damage can release ATP, prostaglandins, cytokines, growth factors, neuropeptides, and other mediators that alter the sensitivity of nociceptors. This phenomenon, known as peripheral sensitization, can increase the responsiveness of sensory neurons during inflammation or injury. The nervous system can subsequently amplify or modify the perception of these signals within the spinal cord and brain.

The relationship between the nervous system and skin extends even further through Skin–Brain Interactions. The skin and brain communicate continuously through neural, endocrine, immune, and molecular pathways. This relationship is often described as part of the brain–skin axis or skin–brain axis. It does not imply that every emotional state directly causes a particular skin condition; rather, it recognizes that physiological signaling between the central nervous system and skin can influence skin function and that signals originating in the skin can, in turn, influence neural activity and perception.

Stress is one of the clearest examples of this bidirectional relationship. Psychological or physiological stress activates the hypothalamic–pituitary–adrenal axis, commonly abbreviated as the HPA axis, as well as the sympathetic nervous system. These systems regulate the release of hormones and neurotransmitters that influence immune activity, inflammation, vascular responses, barrier function, and other physiological processes. The skin itself possesses elements of a local neuroendocrine system capable of producing or responding to several related signaling molecules.

The skin's local neuroendocrine activity includes components analogous to elements of systemic stress-response pathways. Keratinocytes, melanocytes, fibroblasts, immune cells, and other skin cells can respond to neuroendocrine mediators. This local signaling network allows skin to react to environmental and physiological stress independently while remaining connected to systemic regulation.

Stress-related signaling can influence the epidermal barrier. Experimental and clinical research indicates that certain forms of psychological stress can alter epidermal homeostasis, potentially affecting barrier recovery and water loss. Stress can also influence inflammatory signaling and immune responses. These effects demonstrate that the condition of the skin cannot always be explained solely by topical exposures or local tissue events.

The reverse relationship is equally important. Signals generated by the skin can influence the nervous system and subjective experience. Pain, itch, temperature, touch, and inflammation all produce information that travels from peripheral sensory neurons toward the central nervous system. Chronic skin inflammation can therefore have neurological and psychological consequences, particularly when symptoms such as itch or pain persist over long periods.

The relationship between itch and the brain is especially complex. Chronic itch can alter attention, sleep, mood, and behavior, while emotional stress can increase the perception and intensity of itch in susceptible individuals. This creates a feedback loop in which skin inflammation generates sensory signals, the brain interprets those signals, behavioral responses affect the skin, and additional irritation or injury may further amplify the cycle. Scratching is an obvious example: a sensory response intended to relieve itch can produce mechanical injury that perpetuates inflammation and tissue disruption.

Neural signaling also participates directly in wound healing. Sensory nerves can release neuropeptides and other mediators that influence blood vessels, immune cells, keratinocytes, fibroblasts, and tissue regeneration. Experimental evidence indicates that denervation can alter aspects of wound repair, demonstrating that the nervous system is not merely an observer of tissue healing but an active participant.

Hair follicles provide another example of the skin–brain connection. Their growth cycles are influenced by endocrine and neural signals, while follicular structures contain sensory and autonomic nerve fibers. The follicle therefore integrates signals originating from local tissue, systemic hormones, the immune system, and the nervous system. Hair growth and hair loss can consequently be influenced by genetics, hormones, immune activity, aging, nutritional status, medications, and physiological stress.

Sebaceous glands similarly participate in neuroendocrine signaling. Their activity is influenced by hormones and local mediators, while sebocytes can produce and respond to signaling molecules associated with inflammation and stress. The sebaceous unit is consequently part of a larger network connecting metabolism, endocrine function, immunity, microbiology, and neural regulation.

The skin also contains an extensive vascular network that interacts with neural and immune systems. Nerve-derived mediators can influence vascular tone, while inflammatory mediators can alter vascular permeability and blood flow. These interactions contribute to phenomena such as flushing, redness, swelling, and temperature-dependent changes in skin appearance. What appears externally as a change in color can therefore represent the combined activity of blood vessels, nerves, immune mediators, and local tissue signals.

Skin's neurobiology also helps explain why touch has profound physiological effects. Gentle tactile stimulation can activate specific sensory pathways associated with social touch, comfort, and affective processing. The skin is consequently not merely a surface that detects mechanical pressure; it is an interface through which physical contact can influence emotional and neurological states.

The biological connection between skin and brain should not, however, be interpreted as evidence that emotional states are the sole cause of skin disease. Dermatological conditions generally have multifactorial causes involving genetics, immune mechanisms, barrier function, microorganisms, environmental exposure, hormones, neural signaling, and other biological factors. Psychological stress may modify these processes in some circumstances, but it is one component within a much larger system.

The living skin is therefore characterized by continuous communication. Sebaceous glands exchange signals with keratinocytes, immune cells, microorganisms, and hormones. Hair follicles communicate between epithelial and mesenchymal compartments while interacting with nerves, immune cells, and the microbiome. Sensory neurons communicate with nearly every major component of the skin. Neuroendocrine signaling links local tissue activity with systemic physiological states. The brain receives information from the skin while the skin simultaneously responds to signals generated by the brain and peripheral nervous system.

This interconnected architecture means that the visible skin surface is only the outer expression of a much deeper biological system. Sebum reflects glandular activity and endocrine regulation. Hair reflects the cyclical behavior of a complex mini-organ. Itch and pain reflect communication between peripheral nerves, immune mediators, and the central nervous system. Flushing and sweating reveal interactions among autonomic nerves, blood vessels, glands, and environmental conditions. Changes associated with stress can reflect coordinated neuroendocrine and immune responses.

The concept of cutaneous homeostasis therefore extends well beyond barrier maintenance. It includes the regulation of microbial communities, immune surveillance, sensory signaling, glandular secretion, follicular cycling, vascular activity, tissue repair, and communication with systemic physiological networks. Skin remains healthy when these processes are appropriately coordinated and capable of adapting to changing conditions.

The skin is, in this sense, both an organ of protection and an organ of communication. It senses the world, communicates with the brain, interacts with microorganisms, responds to hormones, produces biologically active secretions, participates in immunity, and continually renews its own structures. Its surface may appear still, but beneath that surface is a highly active network of cellular, molecular, neural, endocrine, and microbial activity. Understanding this living architecture is essential to understanding why skin responds to touch, temperature, stress, hormones, inflammation, injury, and environment—and why effective skin science must consider the skin not as an isolated surface, but as a living organ embedded within the entire human biological system.

References

Paus, R. & Cotsarelis, G. The biology of hair follicles. New England Journal of Medicine, 341(7), 491–497, 1999. 

Solanas, G. & Aznar Benitah, S. Regenerating the skin: a task for the heterogeneous stem cell pool and surrounding niche. Nature Reviews Molecular Cell Biology, 14, 737–748, 2013. 

Zhang, B. & Chen, T. Local and systemic mechanisms that control the hair follicle stem cell niche. Nature Reviews Molecular Cell Biology, 25, 87–100, 2024. 

Morita, R., Sanzen, N., Sasaki, H., et al. Tracing the origin of hair follicle stem cells. Nature, 594, 547–552, 2021. 

Blanpain, C. & Fuchs, E. Epidermal homeostasis: a balancing act of stem cells in the skin. Nature Reviews Molecular Cell Biology, 10, 207–217, 2009.

Schneider, M. R., Schmidt-Ullrich, R. & Paus, R. The hair follicle as a dynamic miniorgan. Current Biology, 19(3), R132–R142, 2009.

Sebaceous Glands

Zouboulis, C. C., Schagen, S., Alestas, T., et al. Beyond acne: current aspects of sebaceous gland biology and function. Reviews in Endocrine and Metabolic Disorders, 17, 269–278, 2016. 

Clayton, R. W., Langan, E. A., Ansell, D. M., et al. Neuroendocrinology and neurobiology of sebaceous glands. Biological Reviews, 95(3), 592–624, 2020. 

Niemann, C. & Horsley, V. Development and homeostasis of the sebaceous gland. Seminars in Cell & Developmental Biology, 74, 85–96, 2018. 

Schneider, M. R. & Paus, R. Sebocytes: multifaceted epithelial cells in the skin. Journal of Investigative Dermatology, 133(6), 1353–1356, 2013.

Zouboulis, C. C. Sebaceous gland in human skin: a review. Dermato-Endocrinology, 1(5), 264–270, 2009.

Li, J., Mao, B., Tang, X., et al. Endocrine and metabolic drivers of sebum dysregulation: mechanisms and therapeutic strategies. Life Sciences, 2025. 

Neurobiology of Skin

Misery, L., Brenaut, E., Le Garrec, R., et al. Neuromediators and skin inflammation. Journal of the European Academy of Dermatology and Venereology, 25, 1023–1030, 2011.

Roosterman, D., Goerge, T., Schneider, S. W., Bunnett, N. W. & Steinhoff, M. Neuronal control of skin function: the skin as a neuroimmunoendocrine organ. Physiological Reviews, 86(4), 1309–1379, 2006.

Paus, R., Theoharides, T. C., Arck, P. C., et al. Neuroimmunoendocrine circuitry of the skin. Journal of Investigative Dermatology, 124, 13–17, 2005.

Slominski, A. T., Zmijewski, M. A., Skobowiat, C., Zbytek, B., Slominski, R. M. & Steketee, J. D. Sensing the environment: regulation of local and global homeostasis by the skin's neuroendocrine system. Advances in Anatomy, Embryology and Cell Biology, 212, 1–115, 2012.

Slominski, A. T., Zmijewski, M. A. & Paus, R. How does our skin respond to stress? Nature Reviews Endocrinology, 10, 605–614, 2014.

Skin as a Neuroendocrine Organ

Slominski, A., Wortsman, J., Tuckey, R. C. & Paus, R. Differential expression of HPA axis and related components in the skin. Molecular and Cellular Endocrinology, 265–266, 1–5, 2007.

Slominski, A. T., Zbytek, B., Zmijewski, M. A., et al. Corticotropin releasing hormone and the skin. Frontiers in Bioscience, 12, 5041–5051, 2007.

Slominski, A. T., Manna, P. R. & Tuckey, R. C. Cutaneous glucocorticoidogenesis: securing local homeostasis and the skin's response to environmental stress. Molecular and Cellular Endocrinology, 351, 179–186, 2012.

The skin contains locally produced hormones, neurotransmitters and neuropeptides, together with corresponding receptors, supporting the concept of skin as an autonomous neuroendocrine organ. 

Skin–Brain Interactions

Arck, P. C., Slominski, A., Theoharides, T. C., Peters, E. M. J. & Paus, R. Neuroimmunology of stress: skin takes center stage. Journal of Investigative Dermatology, 126, 1697–1704, 2006.

Peters, E. M. J., Liezmann, C., Spatz, K., et al. Neuroimmune interactions in the skin. Journal of Investigative Dermatology, 134, 1–7, 2014.

Paus, R., Theoharides, T. C. & Arck, P. C. Neuroimmunoendocrine circuitry of the brain–skin connection. Trends in Immunology, foundational literature on the bidirectional communication between neural, endocrine and immune systems.

Slominski, A. T. & Wortsman, J. Neuroendocrinology of the skin. Endocrine Reviews, 29, 1–30, 2008.

Contemporary Brain–Skin Axis Research

The brain–skin relationship is increasingly understood as a genuinely bidirectional neuroendocrine and immune network involving the brain, pituitary and adrenal systems, peripheral nerves, immune pathways and the skin itself. A 2026 narrative review synthesizing 159 studies further emphasizes feedback loops through which stress can influence skin disease while cutaneous inflammation and sensory signaling can, in turn, influence the nervous system. 

Contemporary sebaceous-gland research similarly places sebum production within a broader endocrine, metabolic, neuroendocrine, immune and microbiome network rather than treating the sebaceous gland as an isolated lipid-producing structure. 

These references provide the scientific foundation for 05 — The Living Skin, particularly the concepts of the pilosebaceous unit, follicular stem-cell niches, sebaceous neuroendocrinology, cutaneous sensory biology, and the bidirectional skin–brain axis.

06 — Skin & Environment

The skin exists at the boundary between the internal biological environment of the human body and the constantly changing external environment. Every day, skin encounters ultraviolet radiation, visible light, temperature fluctuations, humidity, wind, air pollutants, particulate matter, microorganisms, chemicals, mechanical friction, water, and changes in atmospheric conditions. At the same time, it responds to internal influences including hormones, circulation, immune activity, metabolism, hydration, nutrition, and the nervous system. Skin biology is therefore shaped by continuous interaction between intrinsic physiology and environmental exposure. The ability of skin to maintain homeostasis depends on its capacity to sense these changes, activate protective mechanisms, repair damage, modify barrier function, regulate inflammation, and adapt its cellular activity.

Environmental Effects on Skin encompass the broad range of biological changes produced or modified by external conditions. Some environmental influences are beneficial or physiologically necessary, while others can become damaging when exposure is excessive, prolonged, or poorly compensated by the skin's protective systems. The same environmental factor may produce different effects depending on its intensity, duration, wavelength, temperature, humidity, anatomical location, age, genetics, barrier condition, and previous exposure history.

Solar radiation is among the most biologically significant environmental influences on skin. The solar spectrum reaching the Earth's surface contains ultraviolet, visible, and infrared radiation. Ultraviolet radiation is conventionally divided into UVA and UVB, according to wavelength. UVA penetrates relatively deeply into the dermis and can contribute substantially to oxidative stress and photoaging. UVB is absorbed more strongly by the epidermis and is particularly important in direct DNA photodamage and erythema. Both wavelengths can influence cellular signaling, immune function, pigmentation, extracellular-matrix remodeling, and carcinogenesis.

The skin has evolved several mechanisms for responding to solar radiation. Melanin absorbs and dissipates portions of ultraviolet energy, reducing the amount reaching sensitive cellular structures. Melanin-containing melanosomes are transferred from melanocytes to keratinocytes, where they can form a protective supranuclear distribution sometimes described as a melanin cap. DNA-repair pathways correct many forms of ultraviolet-induced damage, while antioxidant systems neutralize reactive species generated by radiation. The epidermal barrier and physical architecture of the skin provide additional protection.

These defenses are effective but not absolute. Repeated ultraviolet exposure can produce cumulative molecular damage. UVB can generate direct DNA photoproducts, while UVA can produce substantial reactive oxygen species. These reactive molecules can oxidize lipids, proteins, and DNA and can activate intracellular signaling pathways associated with inflammation and extracellular-matrix degradation. Chronic exposure can therefore alter both the function and structure of skin.

Ultraviolet radiation also modifies immune behavior. Solar radiation can suppress or alter aspects of cutaneous immune surveillance, affecting antigen-presenting cells, cytokine signaling, and other components of the immune system. This immunomodulatory effect has physiological consequences but can also facilitate the persistence of damaged cells and contribute to the development of skin cancers.

Visible light occupies wavelengths between ultraviolet and infrared radiation and can also interact biologically with skin. One of the best-established effects is pigmentation. Visible light, particularly portions of the blue-violet spectrum, can stimulate pigmentation pathways through mechanisms that differ from conventional UVB-induced pigmentation. The response is particularly relevant in individuals with more highly pigmented skin, where visible-light-induced pigmentation can be more persistent.

Infrared radiation contributes primarily to thermal exposure. Excessive heat can influence blood-vessel dilation, inflammatory signaling, oxidative stress, and cellular responses. Chronic environmental heat may therefore interact with other environmental stressors, although the biological effects of infrared exposure are distinct from those of ultraviolet radiation.

Air pollution represents another important component of the modern skin exposome. Particulate matter, polycyclic aromatic hydrocarbons, nitrogen oxides, ozone, and other pollutants can interact with the skin directly or indirectly. Some pollutants can generate oxidative stress at the skin surface, alter barrier function, promote inflammatory signaling, and increase the formation of reactive chemical species.

Particulate matter can accumulate on the skin surface and interact with sebum, sweat, and other components of the surface environment. Certain pollutants can penetrate into superficial skin layers or enter through hair follicles. Pollutant exposure has been associated with oxidative stress, inflammation, pigmentation changes, barrier disruption, and processes relevant to premature skin aging.

Ozone presents an interesting example because it is highly reactive and generally does not need to penetrate deeply into skin to produce biological effects. It can react with surface lipids and generate secondary oxidation products. These products can subsequently participate in inflammatory and oxidative pathways. The skin's surface chemistry therefore becomes an important mediator between atmospheric pollution and cellular responses.

Climate and weather also influence skin physiology. Temperature affects blood flow, sweat production, lipid behavior, enzymatic processes, and water loss. In hot environments, increased sweating and vasodilation contribute to thermoregulation. In cold conditions, cutaneous blood vessels constrict to reduce heat loss. Prolonged exposure to cold can reduce skin perfusion and may contribute to dryness and barrier stress.

Humidity has a particularly direct relationship with epidermal barrier function. Low environmental humidity increases the water gradient between the skin and atmosphere and can increase transepidermal water loss. Prolonged exposure to dry air can therefore contribute to dehydration of the stratum corneum, changes in corneocyte organization, altered enzyme activity, scaling, and increased sensitivity.

High humidity produces a different physiological environment. Increased hydration of the stratum corneum can alter its mechanical properties and the activity of enzymes involved in desquamation. Heat and humidity also increase sweating and can modify the microbial and chemical environment of the skin surface. Environmental humidity therefore does not simply determine whether skin feels “dry” or “oily”; it changes the physical and biochemical conditions under which the barrier operates.

Water exposure itself can influence the stratum corneum. Repeated cycles of wetting and drying can alter corneocyte swelling and barrier organization. Prolonged exposure to water can increase skin hydration temporarily but does not necessarily improve long-term barrier function. Repeated exposure to detergents, solvents, and surfactants can remove or redistribute surface lipids and increase barrier disruption.

Chemical exposure is another major environmental influence. Surfactants, solvents, acids, alkalis, oxidizing agents, preservatives, fragrances, and occupational chemicals can interact with the skin depending on concentration, exposure time, molecular properties, and barrier integrity. Some substances remain primarily on the surface, while others can penetrate through the stratum corneum or enter through appendageal structures.

The skin responds to chemical irritation through coordinated barrier, neural, immune, and inflammatory mechanisms. Keratinocytes can detect chemical stress and release signaling molecules that influence neighboring cells and immune pathways. Sensory nerves can detect irritation and generate sensations such as burning, stinging, or itch. This demonstrates again that the skin is not a passive membrane but a biologically responsive interface.

Mechanical forces also influence skin. Friction, pressure, stretching, compression, scratching, shaving, and repeated movement can produce microscopic injury or stimulate adaptive changes. The skin responds through altered keratinocyte behavior, inflammation, extracellular-matrix remodeling, and changes in tissue thickness. Chronic mechanical stress can therefore contribute to conditions such as callus formation or localized barrier disruption.

The environmental microbiological landscape also interacts with skin. Skin is continuously exposed to microorganisms from air, water, clothing, surfaces, animals, and other humans. The resident microbiome helps maintain ecological balance, while the skin's physical barrier and immune system regulate microbial colonization. Environmental conditions such as humidity, temperature, ultraviolet exposure, sweating, and cleansing can alter the composition and activity of these microbial communities.

The skin's response to the environment is therefore highly adaptive. Barrier function can change, melanogenesis can increase, immune surveillance can be modified, blood flow can change, sweating can increase or decrease, and cellular repair mechanisms can be activated. These responses are not necessarily signs of damage. Many represent normal physiological adaptation. Biological stress becomes more consequential when exposure exceeds the capacity of the tissue to restore homeostasis.

Environmental exposure also accumulates over time. The concept of the exposome describes the totality of environmental exposures experienced by an individual across the lifespan and the biological responses they produce. Skin is particularly useful for understanding the exposome because it directly interfaces with many environmental factors. Ultraviolet radiation, pollution, climate, lifestyle, occupational exposure, cosmetics, cleansing practices, and other environmental variables can interact over years to influence the condition of the tissue.

This cumulative exposure helps explain why two individuals of the same chronological age may have very different skin characteristics. Their histories of ultraviolet exposure, climate, occupation, pollution, barrier disruption, hormonal environment, and other factors may be substantially different. Skin biology therefore cannot be understood exclusively through chronological age or inherited characteristics.

The second major dimension of environmental skin biology concerns variation in pigmentation and other biological characteristics among populations and individuals. Skin Biology Across Different Skin Tones must be understood scientifically without treating skin tone as a simple biological classification of people. Human skin pigmentation exists along a continuous spectrum, produced principally by differences in the quantity, type, distribution, size, persistence, and processing of melanin-containing organelles called melanosomes. Skin tone is therefore a phenotype arising from complex interactions among genetics, melanocyte biology, melanin chemistry, environmental exposure, hormones, inflammation, and other factors.

All humans possess melanocytes in the basal epidermis, and melanocytes generally occur at broadly similar densities across many body sites and across populations. Major differences in visible pigmentation arise primarily from how melanocytes produce, package, distribute, and maintain melanin rather than from simply having dramatically different numbers of melanocytes.

Melanin exists principally in two broad chemical forms: eumelanin, which is brown to black and provides relatively strong photoprotective properties, and pheomelanin, which is yellow to reddish and has different photochemical characteristics. The relative production of these pigments, together with their quantity and distribution, contributes substantially to visible skin coloration.

Melanin synthesis, or melanogenesis, begins within specialized organelles called melanosomes. Enzymes including tyrosinase participate in the biochemical conversion of the amino acid tyrosine through a series of reactions that ultimately produce melanin. Melanosomes mature through distinct developmental stages as pigment is synthesized and deposited within them.

Once formed, melanosomes are transferred from melanocytes into surrounding keratinocytes. The resulting distribution of melanin within the epidermis provides a major component of the skin's visible pigmentation and photoprotective capacity. Differences in melanosome size, maturation, distribution, persistence, and degradation help explain why pigmentation biology differs across skin tones.

In more highly pigmented skin, melanosomes tend to be larger, more individually dispersed within keratinocytes, and more resistant to degradation. In less pigmented skin, melanosomes are generally smaller, more likely to occur in clusters, and are degraded more rapidly within keratinocytes. These differences influence both the optical properties of the skin and the amount of melanin available to absorb and scatter ultraviolet radiation.

Melanin provides meaningful photoprotection, but it is not equivalent to sunscreen. It absorbs ultraviolet radiation and can dissipate much of the absorbed energy as heat, thereby reducing molecular damage. However, even highly pigmented skin remains susceptible to ultraviolet-induced DNA damage, photoaging, and skin cancer. The degree of protection is substantial but incomplete.

Differences in pigmentation also influence the visible consequences of inflammation and injury. One particularly important phenomenon is post-inflammatory hyperpigmentation, in which inflammatory or traumatic events lead to increased or uneven pigmentation after the original process has resolved. This response can occur in all skin tones but is often more persistent and clinically prominent in more highly pigmented skin.

Inflammation can influence melanocyte activity through cytokines, prostaglandins, growth factors, and other signaling molecules. Keratinocytes and immune cells can communicate with melanocytes during inflammatory responses, linking immune activity with pigmentation. Damage to the epidermal basement membrane can also allow melanin-containing material to enter the dermis, where it may be taken up by macrophages and persist for prolonged periods. This process, known as melanin incontinence, can contribute to longer-lasting pigmentation.

Pigmentation also affects the visual assessment of inflammation. Erythema, which results largely from increased cutaneous blood flow and vascular dilation, can appear differently across skin tones. In highly pigmented skin, inflammation may be less visibly red and may instead appear as violaceous, brown, gray, or deeper tonal changes. This does not necessarily indicate a weaker inflammatory response. It can simply reflect the optical interaction between vascular changes, melanin, tissue scattering, and baseline pigmentation.

Similarly, some dermatological conditions can present differently across skin tones. The biological disease process may be similar, while its visible expression differs because melanin modifies the optical appearance of the tissue. Scientific and clinical evaluation therefore benefits from understanding both the underlying biology and the visual properties of different pigmentation levels.

The optical properties of skin are determined by multiple variables, including melanin concentration and distribution, hemoglobin within blood vessels, collagen, water content, tissue thickness, and the way light is absorbed and scattered within the skin. Skin color is consequently not produced by melanin alone. It is an emergent optical characteristic of multiple biological components.

The protective role of melanin is particularly important in environments with high ultraviolet exposure. Evolutionary biology provides evidence that human pigmentation has been shaped in part by historical patterns of ultraviolet radiation. However, modern human populations live across diverse environments and frequently move between geographic regions, making pigmentation biology an important example of adaptation interacting with contemporary environmental exposure.

The relationship between pigmentation and vitamin D biology is also complex. UVB radiation contributes to the cutaneous synthesis of vitamin D, while melanin reduces the penetration of UVB. This does not mean that individuals with darker skin necessarily develop vitamin D deficiency, because vitamin D status is influenced by many factors including latitude, season, clothing, dietary intake, supplementation, age, body composition, and actual sun exposure. Skin pigmentation is one variable within a much larger physiological system.

Environmental effects on skin therefore cannot be separated completely from pigmentation biology. Ultraviolet radiation can stimulate melanogenesis; inflammation can alter pigmentation; pollution can contribute to oxidative stress; climate can alter barrier function; and barrier disruption can modify susceptibility to irritants. The response of skin is produced through the interaction of multiple biological systems rather than a single environmental pathway.

Importantly, variation in skin tone should not be interpreted as a hierarchy of skin quality, health, or biological complexity. Every human skin tone represents a functioning biological phenotype with particular optical, photoprotective, inflammatory, and pigmentary characteristics. Differences are best understood as variations in physiology rather than as differences in biological worth or complexity.

There is also considerable variation within every broad skin-tone category. Two individuals who appear to have similar pigmentation may differ substantially in melanogenesis, tanning response, inflammatory pigmentation, barrier function, sensitivity, aging patterns, and response to environmental exposures. Skin tone therefore provides useful biological information but cannot predict every aspect of an individual's skin behavior.

The most scientifically accurate approach is to treat pigmentation as one component of a continuous biological spectrum and to consider environmental exposure alongside individual genetics, age, anatomical site, hormonal state, barrier condition, immune response, and personal exposure history.

The living skin is consequently an environmental sensor, a protective barrier, a pigmentary organ, an immune interface, and a continuously adapting tissue. Its biology reflects both what occurs inside the body and what the body encounters outside it. Ultraviolet radiation, pollution, climate, humidity, chemicals, mechanical forces, microorganisms, and visible light can all influence cutaneous physiology. At the same time, melanin biology, barrier architecture, immune activity, sensory signaling, and tissue repair determine how skin responds to those environmental pressures.

Understanding environmental skin biology therefore requires moving beyond the idea that the environment simply “damages” skin. The environment is also a source of physiological information to which skin continuously adapts. The biological outcome depends on exposure, dose, duration, susceptibility, protective mechanisms, previous exposure, and the ability of tissue to recover.

Skin tone forms an essential part of this scientific picture because pigmentation changes how skin interacts with light and how certain biological processes become visible. Yet pigmentation is only one component of the extraordinarily complex human skin phenotype. Across the full spectrum of human skin tones, the fundamental architecture of the skin remains shared, while differences in melanin biology, optical properties, inflammatory visibility, pigmentation responses, and environmental adaptation contribute to the remarkable diversity of human skin.

The central principle of Skin & Environment is therefore one of interaction. Skin is shaped by its environment, but it is not passively shaped by it. It senses, adapts, protects, repairs, communicates, and remembers through biological processes operating at molecular, cellular, tissue, and systemic levels. The skin we see is consequently the visible expression of an ongoing dialogue between the organism and the world around it.

References

Passeron, T., Krutmann, J., Andersen, M. L., Katta, R. & Zouboulis, C. C. Clinical and biological impact of the exposome on the skin. Journal of the European Academy of Dermatology and Venereology, 34(S6), 4–10, 2020. 

Krutmann, J., Bouloc, A., Sore, G., Bernard, B. A. & Passeron, T. The skin aging exposome. Journal of Dermatological Science, 85(3), 152–161, 2017.

McDaniel, D., Farris, P. & Valacchi, G. Atmospheric skin aging—contributors and inhibitors. Journal of Cosmetic Dermatology, 18(2), 319–328, 2019.

Moutraji, R. & Taylor, S. C. Skin aging exposome in skin of color populations: Review of the literature. Dermatologic Surgery, 49(3), 272–277, 2023. 

Ultraviolet Radiation, Visible Light & Environmental Photobiology

Sklar, L. R., Almutawa, F., Lim, H. W. & Hamzavi, I. Effects of ultraviolet radiation, visible light, and infrared radiation on erythema and pigmentation: a review. Photochemical & Photobiological Sciences, 12(1), 54–64, 2013. 

Narla, S., Kohli, I., Hamzavi, I. H. & Lim, H. W. Visible light in photodermatology. Photochemical & Photobiological Sciences, 19, 99–104, 2020. 

Ezekwe, N., Maghfour, J. & Kohli, I. Visible light and the skin. Photochemistry and Photobiology, 98(6), 1264–1269, 2022. 

Lim, H. W., Arellano-Mendoza, M. I. & Stengel, F. Current challenges in photoprotection. Journal of the American Academy of Dermatology, 76(3S1), S91–S99, 2017.

Mahmoud, B. H., Ruvolo, E., Hexsel, C. L., et al. Impact of long-wavelength UVA and visible light on melanocompetent skin. Journal of Investigative Dermatology, 130(8), 2092–2097, 2010.

Air Pollution & the Skin

Araviiskaia, E., Berardesca, E., Bieber, T., et al. The impact of airborne pollution on skin: a review. Acta Dermato-Venereologica, 99(2), 123–131, 2019.

Krutmann, J., Liu, W., Li, L. & Pan, X. Pollution and skin: from epidemiological and mechanistic studies to clinical implications. Journal of Dermatological Science, 76(3), 163–168, 2014.

Vierkötter, A. & Krutmann, J. Environmental influences on skin aging and ethnic-specific manifestations. Dermato-Endocrinology, 4(3), 227–231, 2012.

Climate, Temperature & Environmental Stress

Ugwueke, G., Alomary, S. A., Baker, N. J., Taylor, S. C. & Elbuluk, N. The dermatologic effects of climate change on skin of color populations: a comprehensive review. JAAD Reviews, 8, 112–119, 2026. 

The review identifies rising temperatures, worsening air pollution, increased ultraviolet exposure, and extreme weather as important environmental pressures on skin, while emphasizing the limited amount of skin-of-color-specific climate research currently available. 

Human Skin Pigmentation & Melanosome Biology

Quevedo, W. C. Jr., Fitzpatrick, T. B. & Jimbow, K. Human skin color: origin, variation and significance. Journal of Human Evolution, 14(1), 43–56, 1985. 

Jimbow, K., Quevedo, W. C. Jr., Fitzpatrick, T. B. & Szabó, G. Some aspects of melanin biology: 1950–1975. Journal of Investigative Dermatology, 67(1), 72–89, 1976.

Yamaguchi, Y. & Hearing, V. J. Melanocytes and their diseases. Cold Spring Harbor Perspectives in Medicine, 4(5), a017046, 2014.

D'Mello, S. A. N., Finlay, G. J., Baguley, B. C. & Askarian-Amiri, M. E. Signaling pathways in melanogenesis. International Journal of Molecular Sciences, 17(7), 1144, 2016.

Skin Biology Across Different Skin Tones

Taylor, S. C. Skin of color: biology, structure, function, and implications for dermatologic disease. Journal of the American Academy of Dermatology, foundational literature on cutaneous variation and dermatologic manifestations in diverse pigmentation phenotypes.

Bradshaw, M., Taboas, J. M., et al. Reinforcing photoprotection for skin of color: a narrative review. Journal of Drugs in Dermatology, 2023. This review summarizes differences in pigmentation, melanosome biology, ultraviolet responses, visible-light-induced pigmentation, and photoprotection across skin tones. 

Visible Light & Pigmentation in Skin of Color

Narla, S., Kohli, I., Hamzavi, I. H. & Lim, H. W. Visible light in photodermatology. Photochemical & Photobiological Sciences, 19, 99–104, 2020. 

Mahmoud, B. H., Hexsel, C. L., Hamzavi, I. H. & Lim, H. W. Effects of visible light on the skin. Photochemical & Photobiological Sciences, foundational work demonstrating stronger and more persistent visible-light-induced pigmentation in darker skin types. 

Ezekwe, N., Maghfour, J. & Kohli, I. Visible light and the skin. Photochemistry and Photobiology, 98(6), 1264–1269, 2022. 

Visser, W. I., Moola, H., Kannenberg, S., Tod, B. & Lim, H. W. Beyond tint: active ingredient strategies for post-inflammatory hyperpigmentation due to visible light in skin of color. Photodermatology, Photoimmunology & Photomedicine, 42(4), e70112, 2026. 

Photoprotection in Skin of Color

Taylor, S. C. Skin of color: biology, structure, function, and implications for dermatologic disease. Journal of the American Academy of Dermatology.

Al-Niaimi, F. & Chiang, N. Y. Topical antioxidants and photoprotection in darker skin types: current evidence and considerations.

Lim, H. W., Kohli, I., Granger, E., et al. Photoprotection for skin of color: current evidence and clinical considerations.

The contemporary literature emphasizes that greater constitutive pigmentation provides meaningful protection against ultraviolet-induced erythema and some forms of DNA and photodamage, but does not eliminate the need for photoprotection. It also demonstrates that visible light can produce particularly persistent pigmentation in darker skin types. 

Environmental Exposome & Skin of Color

Moutraji, R. & Taylor, S. C. Skin aging exposome in skin of color populations: review of the literature. Dermatologic Surgery, 49(3), 272–277, 2023. 

Passeron, T., Krutmann, J., Andersen, M. L., Katta, R. & Zouboulis, C. C. Clinical and biological impact of the exposome on the skin. Journal of the European Academy of Dermatology and Venereology, 34(S6), 4–10, 2020. 

The exposome framework is particularly valuable for skin biology because it integrates cumulative solar radiation, air pollution, climate, hormones, nutrition, lifestyle, and other environmental influences over the life course rather than examining each exposure in isolation. 

Contemporary Perspective

Recent literature is moving toward a more inclusive environmental model of skin biology. Climate change, increasing heat, air pollution, ultraviolet exposure, and changing environmental conditions may have different dermatological consequences depending on pigmentation phenotype, baseline barrier function, geography, occupation, and cumulative exposure. Importantly, the 2026 literature identifies a significant research gap: people with darker skin tones remain underrepresented in environmental and climate-related dermatology research. 

This makes 06 — Skin & Environment an important concluding article for the foundational Skin Biology sequence: the skin is not biologically isolated from its surroundings, and neither environmental exposure nor pigmentation can be understood adequately without considering the other.