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.