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Features of barrier properties of the skin
Last reviewed: 23.04.2024
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Barrier properties of the skin, which determine the protection of the body from external factors, are provided by a complex interaction of the stratum corneum, highly specialized lipids of the stratum corneum, and also water-lipid mantle.
The stratum corneum provides mechanical protection of the skin from external damages. Due to the constant debridement of the horny scales, the skin surface is cleansed. The thickness of the stratum corneum varies from 9 μm (skin of the eyelids) to 0.5 cm (the skin of the palms and soles) and depends on anatomical localization.
On the surface of the skin, a delicate balance is established between the water content in the stratum corneum and in the environment. To maintain normal hydration of the stratum corneum, there is a unique structure that is rightly compared by many researchers to a brick wall, where the role of "bricks" is performed by corneocytes (postcellular structures of the stratum corneum), and "cement" - highly specialized and uniquely organized intercellular lipids. These lipids include, first of all, ceramides, ceramides, cholesterol, fatty acids, as well as phospholipids, glycosylceramides, free sphingoid bases and cholesterol sulfate.
It is now known that in the stratum corneum of the epidermis there are six main classes of so-called "free", non-corneocyte-bound ceramides and two main classes of ceramides covalently bound to the surface of corneocytes (classes A and B). The composition of ceramides in the horny layer of the skin in humans is very variable and depends on race, concomitant somatic diseases, age, environment and a number of other factors. Ceramides have a rather complex chemical structure. Most of them are long chains of the sphingoid base with the number of carbon atoms from 16 to 22, less often they are represented by dihydrosphingosine, phytosphingosine and 6-hydroxysfingosine. Sphingoid bases are combined with various fatty acids, including free fatty acids, which carry out a number of important biological functions (oleic, linoleic, etc.). The functions of ceramides include not only the retention of water in the skin, but also the regulation of the rate of desquamation, as well as the effect on the differentiation of keratinocytes. It is shown that sphingosine is able to regulate the rate of renewal of the epithelial layer, preventing its rapid replacement without normal differentiation of keratinocytes. Recent studies have shown that sphingosine is also a fairly strong antimicrobial agent, and its presence in the top layer provides protection against microorganisms on the skin surface.
Synthesis of many lipids, including ceramides, is carried out in special organelles of cells of the granular layer - lamellar bodies, in the future these substances form bilipid interlayers between corneocytes. These lipids form the main barrier to water, thus preventing transepidermal water loss (TEWL). They also play the role of a special intercellular cementing substance, which gives the adhesion strength of the postcellular structures of the stratum corneum and ensures the integrity of the skin. The functions of ceramides include not only the retention of water in the skin, but also the regulation of the rate of desquamation, the effect on the differentiation of keratinocytes, and also the antimicrobial effect.
The surface of the skin is covered with a continuous thin water-fat emulsion film - water-lipid mantle. It consists of secretion of sebaceous glands, eccrine sweat glands and ceramides of the stratum corneum. The water-lipid mantle prevents drying of the skin, regulates the desquamation and differentiation of keratinocytes, maintains the constant acidity of the skin surface (pH 4.5-5.5), and is also one of the ways of excretion of metabolic products, medicinal and toxic substances. Depending on the ratio of fat and sweat on the skin, the resulting water-fat emulsion may contain more fat (type "water in oil") or more water (type "oil in water"), which ensures a constant body temperature.
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