The skin barrier is a highly specialized structure that helps keep microbes and toxic chemicals from entering the body and water being lost from the body. But a healthy skin barrier is also key to skin that looks and feels good. Healthy, well-functioning skin is soft, pliable, and smooth, and bounces back after being prodded and poked. Dry, flaky, thin skin and red, itchy rashes may be the result of a poorly functioning skin barrier. And a dysfunctional barrier causes symptoms of skin diseases including eczema, acne, keratosis pilaris, and psoriasis.
We take for granted that our skin is able to keep harmful substances outside of the body and precious moisture inside, all while enduring frequent physical and chemical insults and constantly renewing its smooth, sensitive, waterproof surface. Considering the large area of our skin, the demands put on it, and the energy and materials required to maintain skin structures, it’s not surprising that functionality may sometimes break down.
And we regularly subject our skin to potentially harmful soaps, chlorine, cosmetic ingredients, detergents, hot water, and fragrances. Our lifestyle, beauty, and hygiene routines may be damaging our skin and hindering barrier function. Understanding the effects of skincare, lifestyle, and environmental factors on our skin may help us support a well-functioning skin barrier whether it’s to maintain a healthy, functioning barrier or as part of therapy for skin conditions.
The skin barrier is a highly structured physical barrier made of cells, proteins, and lipids. It has been compared to a brick-and-mortar wall, where the cells are the bricks and the lipids are the mortar. The cells provide structure, and the lipids fill in the cracks, preventing entry or exit of organisms or water. Reinforcement comes from proteins that form rigid filaments, just as rebar steel bars are used for structural support in masonry.
Also contributing to barrier efficacy are compounds that promote immunity, fight bacteria, and help maintain the correct acidity and hydration. Skin cells produce these structural and chemical components. Maintaining the skin barrier is an energy-intensive, and never-ending undertaking – our body is continuously working to maintain skin barrier balance and function. Intact, elastic, protective skin requires good hydration and nutrition. It requires protection from damaging UV radiation and harmful chemicals.
The Cells Of The Skin
The skin barrier is part of the epidermis, the outer layer of skin that is less than a millimeter thick. Underneath the epidermis is the dermis, the thicker layer of skin where nerves, blood vessels, and sweat glands are found.
The primary cell type in the epidermis is the keratinocyte. Keratinocytes are linked to each other—by structures called tight junctions—to form impermeable chains of cells. in addition, keratinocytes make antimicrobial peptides that contribute to the antimicrobial properties of the skin.
The very outermost layer of the epidermis is called the stratum corneum, and it consists of keratinocytes that have transitioned into cells called corneocytes. These cells are linked into a tough network by structures called desmosomes.
After keratinocytes transition into corneocytes, and the cells make the proteins and fats they need, they die. The stratum corneum contains 15 to 20 layers of flattened dead corneocytes, filled with proteins and embedded in a mixture of fatty substances forming a waterproof structure. This mixture of proteins and fats results in an amazing biomaterial that is thin and soft, but also strong and elastic. To continue with the brick wall analogy, it’s as if a brick wall that was reinforced with steel was also flexible.
The Proteins of the Skin
Corneocyte cells contain large amounts of a protein called keratin that binds water, preventing evaporation and keeping the skin hydrated. Another protein in the stratum corneum that's important for a functional barrier is filaggrin. Its name is derived from its ability to aggregate keratin into filaments. When filaggrin binds keratin to form filaments, the cells collapse and flatten into the dense structures found in the stratum corneum.
After it’s completed its work with keratin, filaggrin is repurposed. It’s broken down to individual amino acids that are used to make natural moisturizing factor (NMF), a mixture of water-attracting (humectant) substances that keeps the stratum corneum hydrated and the skin moist.
Natural Moisturizing Factor
NMF is a unique collection of small molecules in the stratum corneum that helps maintain skin hydration and elasticity. Levels of NMF may be low in atopic dermatitis and in very dry skin. And NMF may be lost as a result of bathing or soaking. (Even some skin lipids may be lost from washing with water.)
NMF contains special amino acids like pyrrolidone carboxylic acid (PCA), lactic acid, sugars, urea, and glycerol. These components act as both humectants and osmolytes. Humectants attract water to the skin, from deeper layers of skin or from the air.
Osmolytes help buffer the cells from changes in water content, and are crucial in skin, which may be subject to dry or wet conditions. In very dry conditions, osmolytes will essentially replace water that’s lost, preventing cells from dying.
Urea is an important component of NMF. In addition to supporting skin hydration and elasticity, urea stimulates keratinocytes to grow, make skin barrier proteins such as filaggrin, and produce antimicrobial peptides.
The Lipids of the Skin
The technical term for fatty substances is lipids, and the lipid matrix surrounding the corneocytes contains cholesterol, fatty acids, sterols, waxes, and ceramides. Ceramides play a particularly important role, as each cell is surrounded by a thin layer of this lipid. Ceramides are made of molecules called sphingosines linked to fatty acids.
The lipids in the stratum corneum are uniquely suited to their jobs of waterproofing and providing structure. Ceramide makes up about 50 to 60 percent, and cholesterol and fatty acids each make up about 20 to 25 percent in the barrier. In the right proportions these lipids align in an organized fashion that makes the stratum corneum impermeable. You may hear the term “lamellar structure” to describe how the lipids are lined up in layers.
The fatty acids are not the ones we usually think about. Unlike omega-3, -6, or -9 unsaturated fatty acids in most oils, the ones in skin are largely saturated fats. And for saturated fats they are unusually long, mostly 22 and 24 carbons long. The longer and more saturated fatty acids are, the more rigid they are, and the more hydrophobic, meaning that they repel water. However, small amounts of the polyunsaturated fatty acids alpha-linolenic acid (omega-3) and linoleic acid (omega-6) are also essential for barrier function.
Lipids on the surface of the skin come from the stratum corneum and from sebum, an oily lubricant that helps with water retention and protection from water and UV radiation. It is secreted by sebaceous glands— mostly via hair follicles—and contains waxes, triglycerides (three fatty acids attached to a glycerol molecule), fatty acids, squalene, and cholesterol. Blocked sebaceous glands are involved in acne, and substances that block pores and cause acne are referred to as comedogenic.
Sebum's fatty acids help maintain the slightly acidic pH of skin— from 4.1 to 6—and have antimicrobial activity. In particular, one unique fatty acid, sapienic acid, is important for antimicrobial defense.
Abnormal sebum composition is linked to poor barrier function, as in the skin condition papulopustular rosacea, where skin may be dry and sensitive.
Shedding Skin: Desquamation
The dead corneocytes on the surface of the skin are constantly shed and replaced by keratinocytes produced in the layer underneath. In healthy skin, small areas of corneocytes are continually shed, but in dry skin this process is slowed down and larger areas—that look like scales—are shed, causing roughness and flakiness.
In the skin disease Netherton syndrome, corneocytes are shed too quickly and the skin barrier is severely dysfunctional. This disease is caused by mutations in a gene (SPINK5) that controls the rate at which structures in the stratum corneum are broken down. Symptoms are similar to eczema with inflammation and allergic reactions.
How the Barrier Varies Throughout the Body
There’s huge variability in the skin between body parts. Differences exist in the number of layers of corneocytes, amounts of lipids, the ability to retain water, and the permeability to environmental chemicals. The thickest stratum corneum is on the palms and soles of the feet, and by far the thinnest and most permeable stratum corneum covers the genitals.
The skin on the face is very thin—particularly on the lips and around the eyes—and it has poor barrier function and greater water loss compared to the torso and limbs. On the face, water loss is highest from the skin around the mouth, especially on the upper lip, and from the lips.
And facial skin turns over very quickly. It takes only about a week for the stratum corneum on the face to go through its normal shedding and replacement cycle, as opposed to two weeks for most of the body. This cycle is even shorter in skin conditions such as dandruff and psoriasis.
Because of the very thin stratum corneum on genital tissues, water loss is higher from the vulva than from other parts of the body. Chemicals are absorbed readily through thin skin, so absorption of topically applied substances is extremely high in the genitals.
Absorption of chemicals through the skin also depends on the size of the molecule—the smaller the molecule the more readily it is absorbed. Proteins and hyaluronic acid are examples of very large molecules that are not typically absorbed through intact skin, and steroids and hormones are smaller molecules that may enter the body through the skin.
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This article is for informational purposes only, and is designed to supplement, not to substitute for, consultation with medical professionals. Content is based on scientific research published in peer-reviewed journals, publications from the National Institutes of Health and other medical and scientific organizations, and communications from scientists and licensed healthcare practitioners.
