Anatomo-physiological features of the skin and its appendages

The skin as an organ consists of three layers: the epidermis, the dermis and the subcutaneous fat.

The epidermis is a multilayered flat keratinizing epithelium of the epidermal type. The bulk of the cells are keratinocytes (epidermocytes), and there are also dendritic cells (melanocytes, Langerhans cells, Merkel cells). The epidermis consists of the following layers: basal, spinous, granular, shiny, and horny.

The basal layer is located on the basement membrane, which is 0.7-1.0 μm thick and consists of the following structures: hemidesmosomes (electron-dense areas of the cytoplasmic membrane of epidermocytes, connected with intracellular tonofilaments), shiny, or light, plate (lamina lucida), dense plate (lamina densa), fibroreticular plate (formed by connective tissue fibers of the dermis). Collagen type IV takes part in the construction of the basement 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 and semi-stem cells of the skin are distinguished. The rate of division of basal epidermocytes is not constant, it is subject to daily biorhythms, proportionate to the production of endogenous cortisol by the adrenal glands. There is a complex of exogenous and endogenous factors that accelerate and slow down the division of 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 influencing the rate of proliferation of basal keratinocytes of the epidermis

Factors	They are speeding up	Slow down
Endogenous	The active substance is transforming growth factor-B (TGF-B), estrogens, interleukins and other cytokines, androgens (at the mouth of the sebaceous gland), etc.	Keylones, transforming growth factor-a (TGF-a), interferons and other substances
Exogenous	Phytoestrogens, estrogenic and androgenic drugs, some glycoproteins and proteins of natural and synthetic origin, etc.	Topical glucocorticoids, 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 (pigment dendritic cells, or pigment dendrocytes) 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 Caucasian race, in places of natural pigmentation, the specified cellular elements are also found in the spinous layer. The greatest number of melanocytes in humans is located in the central part of the face and in areas of natural pigmentation (perianal, perigenital areas, areolas of the nipples 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 radiation. Melanocytes differentiate from melanoblasts of neuroectodermal origin. There are no desmosomal connections typical of keratinocytes between melanocytes and keratinocytes. The renewal of melanocytes occurs much more slowly than keratinocytes. Melanocytes produce the pigment melanin. Melanin is synthesized in special organelles of the melanocyte - melanosomes, which are transported to the melanocyte processes. Melanin from the melanocyte processes enters the keratinocytes, where it is located around the nucleus, protecting the nuclear material from ultraviolet rays. Melanin synthesis is regulated by ultraviolet radiation and some hormones (melanocyte-stimulating and ACTH).

Langerhans cells (non-pigmented dendritic cells) are cells of monocyte-macrophage origin (intraepidermal macrophages) responsible for antigen capture, processing, antigen presentation and interaction with T-lymphocytes of the dermis.

Merkel cells (tactile epithelioidocytes) are cells of neural origin that participate in the formation of tactile sensations of the skin. From the dermis side, they are connected to the afferent unmyelinated nerve fiber.

The spinous layer (stratus spinulosum) is represented by 3-15 rows of irregularly shaped cells connected to each other by desmosomes in the area of numerous processes resembling plant spines. Desmosomes are electron-dense areas of the cytoplasmic membrane of epidermocytes connected to intracellular tonofilaments. The number of rows of cells in the spinous layer is not the same in different areas of the skin. Thus, in the skin of the external genitalia, 2 rows of cells in the spinous layer are found, in the skin of the red border of the lips and eyelids - 2-3, folds - 3-4, cheeks and forehead - 5-7, back - 7-8, extensor surface of the elbow and knee joint - 8-10, palms and soles (the 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 (keratohyaline granules). These inclusions contain a protein substance that ensures the process of keratinization of epidermocytes - filaggrin (filament aggregating protein). Filaggrin promotes the aggregation of individual disparate filaments that make up the cytoskeleton of epidermocytes into a single complex. The outcome of such aggregation is the transformation of the cell into a post-cellular structure - a horny scale (horny plate).

The shiny layer (stratus lucidum) is visible only when examined with a light microscope, is present only in the skin of the palms and soles. It consists of 1-2 rows of oxyphilic cells with unclear boundaries and poorly defined organelles. When examined with an electron microscope, it represents the lower rows of the stratum corneum.

The stratum corneum (strains 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.), which provide the basic barrier functions of the skin.

Constant renewal of the epidermis provides the protective function of the skin: due to the rejection of horny scales from the skin surface, it is cleansed of external pollution 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, which are not clearly delineated from each other - the papillary and reticular. The papillary layer is adjacent directly to the epidermis and is represented by loose fibrous connective tissue. This layer provides a connection between the dermis and the basal membrane using reticular and elastic fibers and special anchoring fibrils.

The reticular layer of the dermis is formed by dense, irregular fibrous connective tissue. This layer contains fibrous structures: collagen, elastic and reticular (reticulin, argyrophilic) fibers. Collagen fibers are arranged in a three-dimensional network; they provide, along with hydration of the main substance of the connective tissue of the dermis, skin turgor. These fibrous structures are formed by collagen types I and III. Collagen type I predominates in adults, and type III in children. With age, the production of more hydrophilic collagen, type III, decreases. Elastic fibers responsible for skin elasticity are divided into three types. Thus, directly under the epidermis are the thinnest, most delicate bundles of oxytalan fibers, which are located perpendicular to the skin surface. These fibers are the most sensitive to various trigger factors of the external environment, and they are the first to be destroyed. Deeper and in the dermis, parallel to the skin surface, there are thicker bundles of elaunin and mature (true) elastic fibers. Elaunin and true elastic fibers are oriented along Langer's lines. Due to this orientation of the bundles of elastic fibers, it is recommended to make an incision along Langer's lines during various surgical interventions, which will subsequently ensure the formation of an adequate scar from an aesthetic point of view. It is believed that reticular fibers are precursors of collagen fibers. The dermis contains fibroblasts - cells that produce the ground substance, as well as collagen and elastin proteins, from which collagen and elastic fibers are synthesized in the ground substance of 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 fat is a continuation of the dermis, consists of loose fibrous connective tissue and adipocytes, and has different thickness depending on the state of nutrition and localization. The distribution of subcutaneous fat is regulated by sex hormones. Adipocytes also perform an endocrine function, participating in the synthesis of a number of hormones and releasing factors in different age periods.

The blood supply to the skin is carried out by two arterial and venous plexuses - superficial and deep. The intradermal vascular bed is characterized by some features:

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

The skin microcirculation is a system of microvessels consisting of arterioles, precapillaries, capillaries proper, postcapillaries, venules and lymphatic capillaries. Skin microcirculation 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 (shaped like a "lady's hairpin") contribute to the phenomenon of extravascular shunting, which occurs when the tone of the sympathetic nervous system increases. The highest density of papillary capillaries is found in the skin of the face, red border of the lips, hands and feet.

The deep plexus is formed by a network of larger-caliber vessels in the deep parts of the dermis and subcutaneous fat and is responsible for thermoregulation. Subcutaneous arterial and venous plexuses also participate in thermoregulation. There are anastomoses between the superficial and deep plexuses.

The skin lymphatic system is represented by a superficial network, starting with the papillary sinuses (in the dermal papillae), and a deep network (in the hypodermis), between which the drainage vessels are located. The lymphatic system is closely connected with the skin circulatory system and performs a drainage function.

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

Sebaceous glands are classified as simple alveolar glands, they consist of terminal sections and excretory ducts and are characterized by a holocrine type of secretion. In the overwhelming majority of cases, sebaceous glands are associated with hair follicles, and their ducts open into the mouths of hair follicles. In the skin of the back of the hands and the red border of the lips, there are few sebaceous glands and they are small in size. In the skin of the face (eyebrows, forehead, nose, chin), scalp, midline of the chest, back, armpits, perianal and perigenital areas, the number of sebaceous glands is large - up to 400-900 per cm ², and the glands there are large and multi-lobed. These areas are often affected by seborrhea, acne and seborrheic dermatitis, so they are usually called seborrheic. Sebaceous glands secrete a complex secretion called sebum. Sebum contains free and bound (esterified) fatty acids, a small amount of hydrocarbons, polyhydric alcohols, glycerol, cholesterol and its esters, wax esters, squalene, phospholipids, carotene, and steroid hormone metabolites. Unsaturated fatty acids, which have fungicidal, bactericidal, and virus-static properties, play a special biological role.

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

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

Eccrine sweat glands are located on all areas of the skin. They begin to function from the moment of birth and participate in thermoregulation. They consist of a terminal secretory section and an excretory duct. The terminal section is located in the subcutaneous fat and contains myoepithelial and secretory (light and dark) cells, the activity of the latter is provided by cholinergic fibers. The excretory ducts open freely on the surface of the skin, they are not connected with the hair follicle and are formed by a two-layer cubic epithelium. Eccrine sweat glands produce a hypotonic secretion - sweat with a low content of organic components. When secreting a secretion, the cell remains intact (merocrine secretion).

Apocrine sweat glands are located only in specific areas of the body: the skin of the armpits, the areolas of the nipples of the mammary glands, the perianal and perigenital areas. Sometimes they are found in the skin around the navel and the sacral area. These glands begin to function during puberty. They consist of a terminal secretory section and an excretory duct. The terminal sections are located in the deep parts of the dermis and contain myoepithelial and secretory cells, the activity of the latter is regulated by adrenergic nerve fibers and sex hormones. The accumulation of secretion occurs in the apical part of the secretory cell, which separates into the lumen (apocrine type of secretion). The excretory ducts are formed by a two-layer cuboidal epithelium and flow into the mouth of the hair follicles.

Hair is a keratinized, thread-like appendage of the skin. Adults have up to 2 million hairs on the surface of the body, of which up to 100,000 are on the head. Hair structure is also genetically determined and largely depends on race.

Hair consists of a shaft protruding above the skin level and a root located in the hair follicle, immersed deep into the dermis and subcutaneous fat. The hair follicle is surrounded by a connective tissue hair bursa. Near the skin surface, the hair follicle forms an expansion (funnel), into which the duct of the sebaceous gland (on all areas of the skin) flows, as well as the apocrine sweat gland (in places where these glands are localized). At the end of the follicle there is an expansion - the hair bulb, into which the connective tissue hair papilla with a large number of blood vessels grows. The epithelial cells of the bulb are cambial elements that provide 4 months. It is known that due to the peculiarities of the blood supply, nails grow faster on the right hand of right-handed people, as well as on the II, III and IV fingers. On the feet, the growth rate of the nail plate is somewhat slower, and a healthy nail is renewed on average in 6 months. The growth rate of the nail plate depends on many factors. Thus, nail growth increases during the daytime, in summer, with minor trauma to the nail. In general, the nail plate grows faster in young people than in older people. The nail grows faster in women, especially during pregnancy.

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