Anatomical and physiological features of the skin and its appendages

Skin as an organ consists of three layers: epidermis, dermis and subcutaneous fat.

Epidermis is a multi-layered flat keratinizing epithelium of the epidermal type. The majority of cells are keratinocytes (epidermocytes), there are also dendritic cells (melanocytes, Langerhans cells, Merkel cells). The epidermis consists of the following layers: 6azalnogo, prickly, granular, shiny and horny.

The basal layer is located on the basal membrane, having a thickness of 0.7-1.0 μm and consisting of the following structures: hemidesmosomes (electronically dense sections of the cytoplasmic membrane of the epidermocytes connected to intracellular tonofilaments), a shiny or light lamina (lucida) a dense lamina (lamina densa), a fibreticular plate (formed by connective tissue fibers of the dermis). Collagen type IV is involved in the construction of the basal membrane.

Basal keratinocytes are arranged in a single row and have a cubic or prismatic shape and a large light nucleus. These cells represent the cambial layer of the epidermis: due to their active division, the epithelial layer is constantly replenished. Among the basal cells, stem cells and semisubstituted skin cells are isolated. The rate of division of basal epidermocytes is variable, it is subject to daily biorhythms commensurate with the production of endogenous cortisol by the adrenal glands. There is a complex of exo- and endogenous factors that accelerate and slow down the division of the basal cells of the epidermis. Under normal conditions, a dynamic balance between stimulating and suppressing factors is maintained in the basal layer of the epidermis.

The main factors affecting the rate of proliferation of basal keratinocytes of the epidermis

Factors	Accelerate	Slow down
Endogenous	(transforming growth factor-B, TGF-B), estrogens, interleukins and other cytokems, androgens (at the mouth of the sebaceous-waxy aparete), and others.	Cayons, transforming the growth factor-a (transforming grouth factor, TGF-a). Interferons and other substances
Exogenous	Phytoestrogens, estrogenic and androgenic preparations, some gipocoproteins and proteins of natural and synthetic origin, etc.	Topical glycocorticoids. Cytostatics, interferons and interferonogens, etc.

In the basal layer of the epidermis, in addition to keratinocytes, there are dendritic cells: melanocytes, Langerhans cells, Merkel cells

Melanocytes (pigmentary dendritic cells, or pigmentary dendritic cells) are localized in the basal layer of the epidermis in individuals with white skin. In representatives of the Negroid race, as well as in representatives of the Caucasoid race in places of natural pigmentation, these cellular elements are also found in the prickly layer. The largest number of melanocytes in a person is in the central part of the face and in the areas of natural pigmentation (perianal, perigenital areas, areola of the nipple of the mammary glands). A large number of melanocytes in the centrofacial zone explains the most frequent localization of melasma, a pigment disorder induced by ultraviolet irradiation. Melanocytes differentiate from melanoblasts with neuroectodermal origin. Melanocytes and keratinocytes lack desmosomal connections typical of keratinocytes. Update melanocytes is much slower than keratinocytes. Melanocytes produce melanin pigment. Melanin synthesis is carried out in special melanocyte organelles - melanosomes, which are transported to the processes of melanocyte. Melanin from the processes of melanocyte enters keratinocytes, where it is located around the nucleus, protecting the nuclear material from ultraviolet rays. The synthesis of melanin is regulated by ultraviolet irradiation and some hormones (melanocyte-stimulating and ACTH)

Langerhans cells (pigmentless dendritic cells) are cells of monocyte-macrophage origin (intra-epidermal macrophages), responsible for capture, antigen processing, antigen presentation and interacting with T-lymphocytes of the dermis.

Merkel cells (tactile epithelioidocytes) are cells of a neural origin that participate in the formation of tactile sensations of the skin. From the side of the dermis they are associated with the afferent bezmielinovym nerve fiber.

The thorny layer (stratus spinulosum) is represented by 3-15 rows of irregularly shaped cells connected to each other by desmosomes in the region of numerous processes resembling thorns of plants. Desmosomes are electronically dense sections of the cytoplasmic membrane of the epidermocytes, connected with intracellular tonofilaments. The number of rows of squamous layer cells is not the same in different parts of the skin. Thus, in the skin of the external genital region, 2 rows of spiny layer cells are revealed, in the skin of the red border of the lips and eyelids - 2-3, folds - 3-4, cheeks and forehead - 5-7, backs - 7-8, extensor surface of the ulnar and knee joint - 8-10, palms and soles (so-called "thick skin") - over 10.

The granular layer (stratus granulosum) is represented by 1-3 rows of spindle-shaped cells with a dark nucleus and inclusions in the cytoplasm (keratohyalin granules). These inclusions contain a substance of protein nature, which provides the process of keratinization of the epidermocytes, - filament aggregating protein, which facilitates the aggregation of filaments. Filaggrin promotes the aggregation of individual disparate filaments that make up the cytoskeleton of the epidermocytes into a single complex. The outcome of this aggregation is the transformation of the cell into a postcellular structure - a horny scales (horny plate).

A stratum layer (stratus lucidum) is noticeable only when examined in a light microscope, is presented only in the skin of the palms and soles. It consists of 1-2 rows of oxyphilic cells with fuzzy boundaries and poorly defined organelles. When studying under an electron microscope, it represents the lower rows of the stratum corneum.

The stratum corneum is represented by postcellular structures that do not contain nuclei and organelles (corneocytes). To maintain normal hydration of the stratum corneum, there are highly specialized intercellular lipids (ceramides, free sphingoid bases, glycosylceramides, cholesterol, cholesterol sulfate, fatty acids, phospholipids, etc.) that provide the basic barrier functions of the skin.

The constant renewal of the epidermis provides a protective function of the skin: due to the rejection of the skin of the horny scales, it is purified from external contamination and microorganisms. The epidermis is renewed due to the constant division of basal keratinocytes. The rate of renewal of the epithelial layer depends on the localization, on average it is about 28 days

The dermis consists of two layers, indistinctly delimited from each other, papillary and reticulate. The papillary layer is directly attached to the epidermis and is represented by a loose fibrous connective tissue. This layer provides a link between the dermis and the basal membrane with the help of reticular and elastic fibers and special anchor fibrils.

The mesh layer of the dermis is formed by a dense, unformed fibrous connective tissue. This layer contains fibrous structures: collagen, elastic and reticular (reticulin, argyrophilic) fibers. Collagen fibers are located in the form of a three-dimensional network, they provide, along with the hydration of the main substance of the connective tissue of the dermis, the skin turgor. These fibrous structures are formed by type I and III collagen. Collagen type I prevails in adulthood, and type III - in children. With age, the production of more hydrophilic collagen - type III decreases. Elastic fibers responsible for the elasticity of the skin, are divided into three varieties. So, directly under the epidermis are the thinnest, gentle bunches of oxalate fibers, which are located perpendicular to the surface of the skin. It is these fibers that are most sensitive to various trigger factors of the environment, and they are the first to undergo destruction. Deeper and dermis, parallel to the surface of the skin, there are thicker bundles of elanin and mature (true) elastic fibers. Elaunin and true elastic fibers are oriented along the Langer lines. In connection with this orientation of the bundles of elastic fibers, it is recommended, during various surgical interventions, to make a cut along the Langer lines, which subsequently ensures the formation of an adequate aesthetically pleasing cicatrix. It is believed that the reticular fibers are the precursors of collagen. In the dermis there are fibroblasts - cells that produce the basic substance, as well as collagen and elastin proteins, of which collagen and elastic fibers are synthesized in the main substance of the connective tissue. In addition to fibroblasts, the dermis contains fibrocytes, mast cells, as well as dermal macrophages (histiocytes) and lymphoid cells that carry out local immune surveillance.

Subcutaneous fatty tissue is a continuation of the dermis, consists of loose fibrous connective tissue and adipocytes and has a different thickness depending on the state of nutrition and localization. The distribution of subcutaneous fat is regulated by sex hormones. Apipocytes also perform endocrine function, participating in the synthesis of a number of hormones and releasing factors in different age periods.

Blood supply to the skin is carried out by two arterial and venous plexuses - superficial and deep, The intradermal vascular channel is characterized by some features:

• presence of functional arteriovenous "shunts";
• a high degree of anastomosis between the same and different types of vessels.

The microcirculatory bed of the skin is a microvessel system consisting of arterioles, precapillaries, capillaries proper, post-capillaries, venules and lymphatic capillaries. Microcirculation of the skin is provided by two arteriolar vascular plexuses (subpapillary and subdermal) and three venular vascular plexuses (superficial and deep subpapillary and subdermal). Capillaries located in the papillary layer of the dermis (having the shape of a "ladies' hairpin") contribute to the phenomenon of extraovascular shunting that occurs when the tone of the sympathetic nervous system increases. The highest density of papillary capillaries is found in the face skin, red lip rim, brushes and feet.

The deep plexus is formed by a network of vessels of a larger caliber in the deep sections of the dermis and subcutaneous adipose tissue and is responsible for thermoregulation. Subcutaneous arterial and venous plexuses also participate in thermoregulation. Between the superficial and deep plexus there are anastomoses.

The lymphatic system of the skin is represented by a superficial network beginning with papillary sinuses (in the papilla of the dermis), and a deep network (in the hypodermis), between which are the outflowing vessels. The lymphatic system is closely connected with the circulatory system of the skin and performs the drainage function.

The innervation of the skin is provided by afferent and efferent fibers that form the subepidermal and dermal plexus. The abundance of fibers and nerve endings allows us to characterize the skin as "the basic organ of all perceptions." Efferent fibers innervate smooth muscle tissue of blood vessels, sweat glands and muscles that lift hair. Afferent fibers are associated with encapsulated nerve endings (lamellar Fatera-Pacini bodies, Krause end caps, Ruffini's tactile bodies, Meissner's tactile bodies, Dogel's genital bodies, etc.) located in the dermis and being mechanoreceptors. Afferent fibers are also associated with free ends (nociceptors and thermoreceptors) in the epidermis and dermis.

Sebaceous glands are referred to simple alveolar glands, they consist of end sections and excretory ducts and are characterized by a holocrine type of secretion. In the overwhelming majority of the sebaceous glands are connected with the hair follicles, and their ducts open into the mouth of the hair follicles. In the skin of the region of the rear of the brushes of the red border, the lips of the sebaceous glands are small and they are small in size. In the skin of the face (forehead, forehead, nose, chin), the scalp, midline of the chest, back, axillae, perianal and perigenital region, the quantity of sebaceous glands is large - reaches 400-900 per cm ², with large and multi-lobed glands. These zones are often affected by seborrhea, acne and seborrheic dermatitis, therefore they are commonly called seborrhoeic. Sebaceous glands secrete a complex in composition secret, which is called sebum. The composition of sebum includes free and bound (esterified) fatty acids, in a small amount of hydrocarbons, polyhydric alcohol, glycerin, cholesterol and its esters, waxes, squalene, phospholipids, carotene, and metabolites of steroid hormones. A special biological role is played by unsaturated fatty acids, which possess fungicidal, bactericidal and virosostatic properties.

Secretion of sebum is regulated mainly by hormonal and, to a lesser extent, neurogenic mechanisms. Androgens (testosterone) increase the production of sebum. Interacting with the receptor on the surface of the sebocyte, testosterone under the action of the enzyme 5-alpha-reductase is converted into its active metabolite - dehydrotestosterone, which directly increases secretion production. The amount of biologically active testosterone, the sensitivity of the sebocyte receptors to it, and the activity of 5-alpha-reductase, which determine the secretion rate of the sebaceous glands, are genetically determined. In general, hormonal reflation of sebum secretion can occur at four levels: the hypothalamus, the pituitary gland, the adrenal cortex, and the sex glands. Any change in the hormonal background, leading to changes in the content of androgens, will indirectly affect the secretion of sebum.

Sweat glands are divided into eccrine (simple tubular) and apocrine (simple tubular-alveolar) glands.

Ekkrinnye sweat glands are located on all areas of the skin. Begin to function from the moment of birth and participate in thermoregulation. They consist of an end secretory department and an excretory duct. The end section is located in the subcutaneous adipose tissue and contains myoepithelial and secretory (light and dark) cells, the latter being provided by cholinergic fibers. The outlet ducts open freely on the surface of the skin, they are not connected with the hair follicle and are formed by a two-layered cubic epithelium. Ekkrinnye sweat glands produce a hypotonic secret - sweat with a low content of organic components. When secretion is secreted, the cell retains the whole (the merocrine secretion).

Apocrine sweat glands are located only in special areas of the body; the skin of the axillary cavities, the areola of the nipples of the mammary glands, the perianal, perigenital regions. Sometimes they are found in the skin around the navel, the area of the sacrum. These glands begin to function in the period of puberty. They consist of an end secretory department and an excretory duct. End sections are located in the deep sections of the dermis and contain myoepithelial and secretory cells, the activity of the latter is regulated by adrenergic nerve fibers and sex hormones. Accumulation of secretion occurs in the apical part of the secretory cell that separates into the lumen (the apocrine type of secretion). The inferior ducts are formed by a two-layered cubic epithelium and flow into the mouth of the hair follicles.

Hair is a keratinous filiform appendage of the skin. In adults, there are up to 2 million hairs on the surface of the body, of which up to 100,000 are on the head. The hair structure is also genetically determined and largely depends on the race.

The hair consists of a rod protruding above the level of the skin, and a root located in the hair follicle, immersed deep inside the dermis and subcutaneous fat. The hair follicle is surrounded by a connective tissue bag. Near the surface of the skin, the hair follicle forms an extension (funnel), into which the duct of the sebaceous gland (in all areas of the skin) flows, as well as the apocrine sweat gland (in the localization of these glands). At the end of the follicle there is an extension - a hair bulb, into which a connective tissue hair papilla grows with a large number of blood vessels. Epithelial cells of the bulb are cambial elements that provide 4 months. It is known that in connection with the peculiarities of the blood supply, the nails grow faster on the right hand with right-handed ones, and also on the II, III and IV fingers. On the feet, the growth rate of the nail plate is somewhat smaller, and a healthy nail is updated on average for 6 months. The growth rate of the nail plate depends on many factors. So, the growth of the nail is intensified in the daytime, in the summer, with minor nail trauma. In general, in young people, the nail plate grows faster than in the elderly. The nail grows faster in women, especially during pregnancy.

[1], [2], [3], [4], [5]