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Lecture notes, cheat sheets
Histology. Leather and its derivatives
Directory / Lecture notes, cheat sheets Table of contents (expand) Topic 23. LEATHER AND ITS DERIVATIVES The skin forms the outer covering of the body, the area of which in an adult reaches 1,5 - 2 m2. Of the appendages of the skin, a person has hair, nails, sweat and sebaceous glands. Leather The function of the skin is to protect the underlying parts of the body from damage. Healthy skin is impervious to microorganisms, many poisonous and harmful substances. The skin is involved in water and heat exchange with the external environment. During the day, about 500 ml of water is excreted through the human skin, which is 1% of its total amount in the body. In addition to water, various salts, mainly chlorides, as well as lactic acid and products of nitrogen metabolism, are excreted through the skin with sweat. About 82% of all body heat loss occurs through the skin surface. In cases of violation of this function (for example, during prolonged work in rubber overalls), overheating of the body and heat stroke may occur. Vitamin D is synthesized in the skin under the action of ultraviolet rays. Its absence in the body causes rickets, a serious disease. The skin is in a certain ratio with the sex glands of the body. As a result, most of the secondary sexual characteristics appear in the skin. The presence in the skin of an abundant vascular network and arteriolo-venular anastomoses determines its significance as a blood depot. In an adult, up to 1 liter of blood can linger in the vessels of the skin. Due to the abundant innervation, the skin appears as a receptor field, consisting of tactile, temperature and pain nerve endings. In some areas of the skin, for example, on the head and hands, 1 cm2 its surface has up to 300 sensitive points. skin development The two main components of the skin have different origins. The epidermis develops from the ectoderm, and the skin itself develops from the mesenchyme. development of the epidermis. The early embryo is covered with a single layer of ectodermal cells. At the beginning of the 2nd month of development, flat surface cells and the underlying basal layer of cuboidal epithelial cells responsible for the formation of new cells are distinguished in the emerging epidermis. Later, an intermediate layer forms between the superficial and basal layers. By the end of the 4th month in the epidermis, the basal layer, a wide layer of spiny cells, granular and stratum corneum are distinguished. During the first 3 months of development, migrants from the neural crest colonize the epidermis. Later, cells of bone marrow origin appear. The development of the skin itself. The skin itself (dermis) is of mesenchymal origin. Its formation involves cells that migrate from the somite dermatome. On the 3rd - 4th month, outgrowths of connective tissue protruding into the epidermis are formed - papillae of the skin. Lubrication of the skin. The skin of the fetus is covered with a white lubricant, consisting of the secretion of the sebaceous glands, fragments of epidermal cells and hair. The lubricant protects the skin from the effects of amniotic fluid. Structure The skin consists of two parts - epithelial and connective tissue. The epithelium of the skin is called the cuticle (or epidermis), and the connective tissue base is called the dermis (or the skin itself). The connection of the skin with the underlying parts of the body occurs through a layer of adipose tissue - subcutaneous tissue (or hypodermis). The thickness of the skin in different parts of the body varies from 0,5 to 5 mm. The epidermis is composed of keratinized squamous epithelium. Its thickness is from 0,03 to 1,5 mm or more. The thickest epidermis on the palms and soles, consisting of many layers of cells. These cells consist of 5 main layers, which include basal, spiny, granular, shiny and horny. Directly on the basement membrane, which separates the epithelium from the dermis, are the cells that make up the basal layer. Among them, basal epidermocytes, melanocytes (pigment cells) are distinguished, the quantitative ratio between which is approximately 10: 1. The shape of basal epidermocytes can be cylindrical or oval, with the presence of basophilic cytoplasm and a rounded nucleus saturated with chromatin. They revealed organelles of general importance, tonofibrils and granules of dark brown or black pigment (melanin). Their connection with each other and with overlying cells occurs through desmosomes, and with the basement membrane - through hemidesmosomes. Melanocytes on preparations stained with hematoxylineosin have the appearance of light cells. Melanocytes do not have desmosomes and lie freely. Their cytoplasm contains large amounts of melanin grains, but organelles are poorly developed and tonofibrils are absent. Above the basal cells in 5-10 layers are polygonal-shaped cells forming a prickly layer. Numerous short cytoplasmic processes ("bridges") are clearly visible between the cells, at the meeting point of which there are desmosomes. Desmosomes end with tonofibrils. In addition to epidermocytes, white process cells (Langerhans cells) are observed in the spinous layer. They lack tonofibrils and do not form desmosomes. There are many lysosomes in their cytoplasm, and there are melanin granules captured from the processes of melanocytes. Currently, many authors regard these cells as epidermal macrophages migrating into the epidermis from the mesenchyme during embryogenesis. A feature of the basal and deep levels of the spinous layer of the epidermis is the ability of epidermocytes to reproduce by mitotic division. Therefore, they are often combined under the name of the germinal layer. Thanks to him, the renewal of the epidermis occurs in various parts of the human skin within 10 - 30 days (physiological regeneration). The granular layer consists of 3-4 layers of relatively flat cells. Their cytoplasm contains ribosomes, mitochondria, lysosomes and their variety - keratinosomes (in the form of layered bodies), as well as bundles of fragmented tonofibrils and large keratohyalin granules lying next to them. Staining of granules occurs through the use of basic dyes, consisting of polysaccharides, lipids and proteins, characterized by a high content of basic amino acids (proline, arginine), as well as a sulfur-containing amino acid (cystine). The presence in the cells of the granular layer of the complex of keratohyalin with tonofibrils indicates the beginning of keratinization processes, since, according to many authors, it is the initial stage in the formation of keratin (keratin). The next layer (shiny) also consists of 3-4 layers of flat cells, in which the nuclei cease to stain due to their death, and the cytoplasm is diffusely impregnated with a protein substance - eleidin, which, on the one hand, is not stained with dyes, and on the other hand, refracts light well . Because of this, the structure of cells in the shiny layer of the border is imperceptible, and the entire layer looks like a shiny stripe. It is believed that eleidin is formed from the proteins of tonofibrils and keratohyalin by oxidation of their sulfhydryl groups. Eleidin itself is regarded as a precursor of keratin. The stratum corneum is represented by many horny scales. The scales contain keratin and air bubbles. Keratin is a protein rich in sulfur (up to 5%), characterized by resistance to various chemical agents (acids, alkalis, etc.). Inside the cells are keratin fibrils. In rare cases, there are remains of tonofibrils, representing a delicate network and a cavity formed at the site of the dead nucleus. The horny scales that are on the surface are constantly falling off, sloughing off and being replaced by new ones coming from the layers lying below. During desquamation, keratinosomes are of great importance, which leave the cells, concentrating in the intercellular spaces. As a result, lysis (dissolution) of desmosomes and separation of horny cells from each other is observed. The value of the stratum corneum is determined by the fact that it has great elasticity and poor thermal conductivity. Thus, a number of cell components are involved in the process of keratinization of the epidermis of the skin: tonofibrils, keratohyalin, keratinosomes, desmosomes. Compared to the skin of the palms and soles, the epidermis is much thinner in other areas of the skin. Its thickness, for example, on the scalp does not exceed 170 microns. The shiny layer is absent in it, and the horny layer is represented by only 2-3 rows of keratinized cells (scales). In all likelihood, keratinization in this case proceeds according to a shortened cycle. Consequently, most of the skin has an epidermis, which consists of 3 main layers - sprout, granular and horny. Moreover, each of them is much thinner than the corresponding layers of the epidermis of the skin of the palms and soles. Under the influence of some external and internal factors, the nature of the epidermis can change significantly. So, for example, with strong mechanical influences, with A-avitaminosis, under the influence of hydrocortisone, the processes of keratinization sharply increase. The concept of a proliferative unit. A proliferative unit is a clone that combines different stages of differon, cells of different degrees of differentiation and originating from a single stem cell located in the basal layer and in contact with the basement membrane. As cells differentiate, they move to the surface of the layer. Differentiation. The stem cell is in contact with the basement membrane. As cells differentiate and multiply, they move to the surface of the epidermis, forming together a proliferative unit of the epidermis, which, in the form of a column, occupies a certain area of it. Keratinocytes that have completed their life cycle are exfoliated from the surface of the stratum corneum. Proliferative unit - a structure formed by keratinocytes of different layers of the epidermis, of varying degrees of differentiation and originating from one stem cell of the basal layer. The nature of the population. Keratinocytes are referred to as a renewing cell population. Their maximum mitotic activity is observed at night, and life expectancy is 2 - 4 weeks. The concept of hard and soft keratin. By physical and chemical properties, hard and soft keratin are distinguished. Solid keratin is present in the cortex and cuticle of the hair. This type of keratin is found in human hair and nails. It is more durable and chemically more resistant. Soft keratin is the most abundant, present in the epidermis, localized in the hair medulla and in the inner root sheath, and contains less cystine and disulfide bonds than hard keratin. Influence of hormones and growth factors on the layers of the epidermis. Keratinocytes serve as targets for numerous hormones and growth factors. The epidermal growth factor (EGF), keratinocyte growth factor, fibroblast growth factor, growth factor FGF7, transforming growth factor (TGFoc), which stimulate keratinocyte mitoses, are of the greatest importance. Substance P, released from the terminals of sensitive nerve fibers, has a similar effect. 1a,25-dihydroxycholecalciferol inhibits secretion and DNA synthesis in keratinocytes and stimulates terminal differentiation. Application: 1a,25-dihydroxycholecalciferol is used in psoriasis, when the process of differentiation of keratinocytes is disturbed and their proliferation is enhanced, it gives a positive therapeutic effect. melanocytes. Melanocytes are located in the basal layer, their number varies significantly in different areas of the skin. Melanocytes originate from the neural crest and synthesize pigments (melanins) enclosed in special vesicles - melanosomes. Tyrosinase. Melanocytes are characterized by a copper-containing and ultraviolet-sensitive enzyme - tyrosinase (tyrosine hydroxylase), which catalyzes the conversion of tyrosine to DOPA. Insufficiency of tyrosinase or its blocking in melanocytes leads to the development of various forms of albinism. Melanosomes. Tyrosinase after synthesis on the ribosomes of the granular endoplasmic reticulum enters the Golgi complex, where it is "packed" into vesicles, which then merge with premelanosomes. Melanin is produced in melanosomes. DOPA is oxidized by DOPA oxidase and converted into melanin during chemical reactions. The histochemical reaction to DOPA makes it possible to identify melanocytes among other skin cells. Melanin. Long processes of melanocytes go into the spiny layer. Melanosomes are transported along them, the contents of which (melanin) are released from melanocytes and captured by keratinocytes. Here, melanin undergoes degradation under the action of lysosome enzymes. Melanin protects the underlying structures from exposure to ultraviolet radiation. The acquisition of a tan indicates an increase in the production of melanin under the influence of ultraviolet radiation. There are two types of melanins in human skin - eumelanin (black pigment) and pheomelanin (red pigment). Eumelanin is a photoprotector, pheomelanin, on the contrary, can contribute to ultraviolet damage to the skin due to the formation of free radicals in response to irradiation. People with brown (red) hair, light eyes, and skin contain predominantly pheomelanin in their hair and skin, have a reduced ability to produce eumelanin, develop a slight tan, and are at risk of UV overexposure. Melanocortins. Of the melanocortins, α-melanotropin regulates the ratio of eumelanin and pheomelanin in the skin. In particular, α-melanotropin stimulates the synthesis of eumelanin in melanocytes. Specific agouti protein blocks the action of melanotropins through melanocortin receptors, which helps to reduce the production of eumelanin. Langerhans cells. They make up 3% of all epidermal cells. These antigen-presenting cells carry class I and class II MHC proteins on the cell membrane and are involved in the immune response. They originate from the bone marrow and belong to the mononuclear phagocyte system. Differentiation of Langerhans cells from CD34+ pluripotent stem cells is supported by TGFβ1, TNFα and GM-CSF. In the epidermis, these cells are located mainly in the spinous layer. The cells contain an irregularly shaped nucleus with invaginations, a moderately developed granular endoplasmic reticulum, a Golgi complex, a small number of microtubules, and elongated Birbeck cytoplasmic granules with longitudinal striation. The Langerhans cell marker is the glycoprotein langerin. Actually the skin, or dermis, has a thickness of 0,5 to 5 mm, the largest - on the back, shoulders, hips. The dermis consists of 2 layers (papillary and reticular), which do not have a clear boundary between them. The papillary layer is located directly under the epidermis and consists of loose fibrous unformed connective tissue responsible for trophic function. This layer was named due to the presence of numerous papillae protruding into the epithelium. The various parts that make up the skin vary in size and quantity. The main part of the papillae (up to 0,2 mm high) is concentrated in the skin of the palms and soles. Facial papillae are poorly developed and may disappear with age. The pattern on the surface of the skin is determined by the papillary layer of the dermis, which has a strictly individual character. The connective tissue of the papillary layer consists of thin collagen, elastic and reticular fibers, cells with the most common fibroblasts, macrophages, tissue basophils (mast cells), etc. In addition, there are smooth muscle cells, in some places collected in small bundles. Many of them are related to the muscles that raise the hair, but there are muscle bundles that have no connection with them. A particularly large number of them are concentrated in the skin of the head, cheeks, forehead and dorsal surface of the limbs. The reduction of these cells causes the appearance of the so-called goose bumps. At the same time, blood flow to the skin decreases, as a result of which the heat transfer of the body decreases. The reticular layer consists of a dense, irregular connective tissue with powerful bundles of collagen fibers running either parallel to the skin surface or obliquely, and a network of elastic fibers. Together they form a network where, by means of the functional load on the skin, its structure is determined. In areas of the skin that experience strong pressure (skin of the foot, fingertips, elbows, etc.), a wide-looped, rough network of collagen fibers is well developed. In the same areas where the skin is significantly stretched (the area of the joints, the back of the foot, the face, etc.), there is a narrow-loop collagen network in the mesh layer. The course of elastic fibers basically coincides with the course of collagen bundles. Their number predominates in areas of the skin that are often stretched (in the skin of the face, joints, etc.). Reticular fibers are found in small numbers. They are usually found around blood vessels and sweat glands. The cellular elements of the reticular layer are represented mainly by fibroblasts. In most parts of the human skin, its reticular layer contains sweat and sebaceous glands, as well as hair roots. The structure of the mesh layer is fully consistent with its function - to ensure the strength of the entire skin. Bundles of collagen fibers from the reticular layer of the dermis pass into the layer of subcutaneous tissue. Between them there are significant gaps filled with lobules of adipose tissue. Subcutaneous tissue softens the effect of various mechanical factors on the skin, so it is especially well developed in places such as fingertips, feet, etc. Here, complete preservation of subcutaneous tissue is observed, despite the extreme degree of exhaustion of the body. In addition, the subcutaneous layer provides some mobility of the skin compared to the underlying parts, which leads to its protection from ruptures and other mechanical damage. Finally, subcutaneous tissue is the most extensive fat depot of the body, and also provides its thermoregulation. Skin pigment, with very few exceptions, is found in the skin of all people. People whose body is devoid of pigment are called albinos. Skin pigment belongs to the group of melanins. Melanin is formed during the oxidation of the amino acid tyrosine under the influence of the enzyme tyrosinase and DOPA oxidase. In the skin dermis, the pigment is located in the cytoplasm of dermal melanocytes (process-shaped cells), however, unlike epidermal melanocytes, they do not give a positive DOPA reaction. Because of this, the pigment cells of the dermis contain but do not synthesize the pigment. How the pigment enters these cells is not exactly known, but it is assumed that it comes from the epidermis. Dermal melanocytes are of mesenchymal origin. Relatively often they are found only in certain places of the skin - in the anus and in the areola. Pigment metabolism in the skin is closely related to the content of vitamins in it, and also depends on endocrine factors. With a lack of B vitamins, melanogenesis in the epidermis decreases, and a lack of vitamins A, C and PP causes the opposite effect. Hormones of the pituitary, adrenal, thyroid and sex glands have a direct effect on the level of melanin pigmentation of the skin. Blood vessels are involved in the formation of plexuses in the skin, from which the news depart, participating in the nutrition of its various parts. The vascular plexuses are located in the skin at different levels. There are deep and superficial arterial plexuses, as well as one deep and two superficial venous plexuses. Skin arteries originate from a wide-loop vascular network located between the muscular fascia and subcutaneous fatty tissue (fascial arterial network). Vessels depart from this network, which, after passing through the layer of subcutaneous adipose tissue, branch out, forming a deep skin arterial network, from which there are branches involved in the blood supply to the fatty lobules, sweat glands and hair. From the deep skin arterial network, arteries begin, which, after passing through the reticular layer of the dermis at the base of the papillary layer, break up into arterioles involved in the formation of the subpapillary (superficial) arterial network, from which branches branch, which in the papillae break up into capillaries, shaped like hairpins not long more than 0,4 mm. Short arterial branches extending from the subpapillary network supply blood to the papillary groups. It is characteristic that they do not anastomose with each other. This may explain why sometimes redness or blanching of the skin occurs in patches. From the subpapillary network, arterial vessels branch towards the sebaceous glands and hair roots. The capillaries of the papillary layer, sebaceous glands and hair roots are collected in veins that flow into the subpapillary venous plexus. There are two papillary plexuses, lying one after the other, from which the blood is directed to the skin (deep) venous plexus, lying between the dermis and subcutaneous fatty tissue. Blood is sent to the same plexus from the fat lobules and sweat glands. The connection of the skin plexus with the fascial occurs through the venous plexus, from which larger venous trunks depart. Arteriovenular anastomoses (glomus) are widespread in the skin, especially numerous at the tips of the fingers and toes and in the area of the nail bed. They are directly related to the process of thermoregulation. The lymphatic vessels of the skin form two plexuses - a superficial one, lying slightly below the subpapillary venous plexus, and a deep one, located in the subcutaneous fatty tissue. The innervation of the skin occurs both through the branches of the cerebrospinal nerves and through the nerves of the autonomic system. The cerebrospinal nervous system includes numerous sensory nerves that form a huge number of sensory nerve endings in the skin. The nerves of the autonomic nervous system innervate blood vessels, smooth myocytes and sweat glands in the skin. Nerves in the subcutaneous adipose tissue form the main nerve plexus of the skin, from which numerous stems depart, which play a major role in the creation of new plexuses located around the hair roots, sweat glands, fatty lobules and in the papillary dermis. The dense nerve plexus of the papillary layer is involved in the transfer to the connective tissue and to the epidermis of myelinated and unmyelinated nerve fibers involved in the formation of many sensory nerve endings that are unevenly distributed in the skin. A large number of them are observed in areas of the skin with hypersensitivity, for example, on the palms and soles, on the face, in the genital area. They are also a large group of non-free nerve endings, such as lamellar nerve bodies, terminal flasks, tactile bodies, genital bodies and tactile discs. It is believed that the feeling of pain is transmitted by free nerve endings located in the epidermis, reaching the granular layer, as well as by nerve endings lying in the papillary dermis. The sense of touch (touch) is perceived by the tactile bodies and discs, as well as the nerve plexuses (cuffs) of the hair. The first are located in the papillary layer of the dermis, the second - in the germ layer of the epidermis. Nerve cuffs are nerve networks that wrap around the hair roots to the level at which the sebaceous glands are located. In the epidermis, in addition, there are tactile cells (Merkel cells) that are in contact with the tactile discs. These are large, round or elongated cells with a light vacuolated cytoplasm, in which osmophilic granules are present. Merkel cells are thought to be of glial origin. The feeling of pressure is associated with the presence of lamellar nerve bodies in the skin. These are the largest nerve endings (up to 2 mm in diameter) that lie deep in the skin. The feeling of warmth is probably perceived by free nerve endings, and the feeling of cold by Merkel cells. Hair Hair covers almost the entire surface of the skin. The highest density of their location is on the head, where their total number can reach 100 thousand. The length of the hair varies from a few millimeters to 1,5 m, the thickness is from 0,005 to 0,6 mm. There are three types of hair: long (hair of the head, beard, mustache, and also located in the armpits and on the pubis), bristly (hair of the eyebrows, eyelashes, and also growing in the external auditory canal and on the eve of the nasal cavity); vellus (hair covering the rest of the skin). Structure. Hair is an epithelial appendage of the skin. There are two parts in the hair - the shaft and the root. The hair shaft is above the surface of the skin. The hair root is hidden in the thickness of the skin and reaches the subcutaneous fatty tissue. The hair shaft is formed by the cortex and cuticle. The root of long and bristly hair consists of cortical substance, medulla and cuticle, in vellus hair - only of cortical substance and cuticle. The hair root is located in the hair follicle (or follicle), the wall of which consists of the inner and outer epithelial (root) sheaths and the connective tissue hair follicle. The hair root ends with an extension (hair follicle). Both epithelial sheaths merge with it. From below, connective tissue with capillaries in the form of a hair papilla protrudes into the hair follicle. At the point of transition of the hair root to the shaft, the epidermis of the skin forms a small depression - a hair funnel. Here, the hair, coming out of the funnel, appears above the surface of the skin. The growth layer of the funnel epidermis passes into the outer epithelial sheath. The internal epithelial sheath ends at this level. The duct of one or more sebaceous glands opens into the hair funnel. Below the sebaceous glands in an oblique direction passes the muscle that lifts the hair. The hair follicle is the hair matrix, that is, the part of the hair from which it grows. It consists of epithelial cells capable of reproduction. Reproducing, the cells of the hair follicle move into the medulla and cortex of the hair root, its cuticle and into the inner epithelial sheath. Thus, due to the cells of the hair follicle, the growth of the hair itself and its inner epithelial (root) sheath occurs. The hair follicle is nourished by vessels located in the hair papilla. As the cells of the hair bulb pass into the medulla and cortex, into the hair cuticle and the inner epithelial sheath, they move further and further away from their source of nutrition - from the vessels of the hair papilla. In this regard, irreversible changes and the processes of keratinization associated with them slowly increase in them. In areas more distant from the hair bulb, the cells die and turn into horny scales. Therefore, the structure of the hair root, its cuticle and the inner epithelial sheath is not the same at different levels. The process of keratinization of cells occurs most intensively in the cortex and cuticle of the hair. As a result, hard keratin is formed in them, which differs in physical and chemical properties from soft keratin. Hard keratin is more durable. In humans, nails are also built from it. Hard keratin is poorly soluble in water, acids and alkalis, it contains more sulfur-containing amino acids cystine than in soft keratin. During the formation of solid keratin, there are no intermediate stages - the accumulation of keratohyalin and eleidin grains in the cells. In the medulla of the hair and the inner epithelial sheath, keratinization processes proceed in the same way as in the epidermis of the skin, i.e., keratohyalin (trichogialin) grains first appear in the cells, which then turn into soft keratin. The medulla of the hair is well expressed only in long and bristly hair. It is absent in vellus hair. The medulla consists of polygonal-shaped cells lying on top of each other in the form of coin columns. They contain acidophilic shiny granules of trichohyalin, small gas bubbles and a small amount of pigment grains. The pigment is formed in the hair follicle by melanocytes, which are located directly around the hair papilla. The processes of keratinization in the medulla proceed slowly, therefore, approximately to the level of the ducts of the sebaceous glands, the medulla consists of incompletely keratinized cells, in which compacted nuclei or their remains are found. Only above this level, the cells undergo complete keratinization. Trichohyalin differs from keratohyalin in that it is stained not with basic, but with acidic dyes. With age, the processes of keratinization in the medulla of the hair intensify, the amount of pigment in the cells decreases and the number of air bubbles increases - the hair turns gray. The cortical substance of the hair makes up its bulk. The processes of keratinization in the cortical substance proceed intensively and without intermediate stages. Throughout most of the root and the entire hair shaft, the cortical substance consists of flat horny scales. Only in the region of the neck of the hair bulb in this substance are not completely keratinized cells with oval nuclei found. The horny scales contain hard keratin, the remains of nuclei in the form of thin plates, pigment grains and gas bubbles. The better the cortical substance is developed in the hair, the stronger, more elastic and less brittle it is. By old age, in the horny scales of the cortical substance, as in the medulla, the number of gas bubbles increases. The hair cuticle is directly adjacent to the cortex. Closer to the hair follicle, it is represented by cylindrical cells lying perpendicular to the surface of the cortex. In the more superficial areas of the hair root, these cells acquire an inclined position and turn into horny scales, overlapping each other in the form of tiles. These scales contain hard keratin, but are completely devoid of pigment and the remainder of the nuclei. The internal root sheath is a derivative of the hair follicle. In the lower sections of the hair root, it passes into the substance of the hair bulb, and in the upper sections at the level of the ducts of the sebaceous glands it disappears. In the lower parts of the internal root sheath, three layers are distinguished: the cuticle, the granular epithelial layer (Huxley's layer), and the pale epithelial layer (Henle's layer). In the middle and upper sections of the hair root, all these 3 layers merge, and here the inner root sheath consists only of completely keratinized cells containing soft keratin. The outer root sheath is formed from the germ layer of the epidermis of the skin, which continues up to the hair follicle. At the same time, it gradually becomes thinner and at the point of transition to the hair follicle consists of only 1 - 2 layers of cells. The cells have a light vacuolated cytoplasm due to the presence of a significant amount of glycogen in it. The hair follicle is the connective tissue sheath of the hair. It distinguishes the outer longitudinal layer of fibers, the inner and circular layers of fibers and the basement membrane. The raising hair muscle is made up of smooth muscle cells. In bristly, vellus hair, beard hair and armpits, it is absent or poorly developed. The muscle lies in an oblique direction and is woven into the hair follicle of the hair at one end, and into the papillary dermis with the other. When it is reduced, the root takes a perpendicular direction to the surface of the skin and as a result of this, the hair shaft rises slightly above the skin (the hair stands on end). Muscle contraction also causes some compression of the skin and the blood vessels lying in its upper layers (goosebumps). As a result, the body's heat transfer through the skin is reduced. Hair change. The lifespan of a hair is from several months to 2-4 years, so there is a periodic change of hair throughout life. This process consists in the fact that the hair papilla of the hair is reduced, the cells in the hair follicle lose their ability to multiply and undergo keratinization, which leads to the formation of the so-called hair bulb, and hair growth stops. The hair flask is separated from the hair papilla and, along the case formed by the outer root sheath, moves upward to the attachment site of the muscle that lifts the hair. In this place, a small invagination is formed in the wall of the hair follicle - the hair bed. A hair flask is placed in it. The desolate part of the epithelial sheath collapses and turns into a cell cord. At the end of this strand, the hair papilla subsequently re-forms. It grows into the end of the epithelial cord and gives rise to a new hair follicle. This is where the new hair starts to grow. The new hair grows along the epithelial strand, which at the same time turns into its outer epithelial sheath. As the new hair grows further, it displaces the old hair from its hair bed, and the process ends with the loss of the old and the appearance of a new hair on the surface of the skin. Nails Nails are a derivative of the epidermis of the skin. They develop in the 3rd month of the intrauterine period. Before the nail appears, the so-called nail bed is formed at the site of its future bookmark. At the same time, the epithelium covering the dorsal surfaces of the terminal phalanges of the fingers and toes thickens and somewhat sinks into the underlying connective tissue. In a later stage, the nail itself begins to grow from the epithelium of the proximal part of the nail bed. Due to slow growth (about 0,25 - 1 mm per week), only by the last month of pregnancy does the nail reach the tip of the finger. Nail - a dense horny plate lying on the nail bed. The nail bed from the sides and at the base is limited by skin folds (or nail folds), posterior and lateral. Between the nail bed and the nail folds there are nail gaps (posterior and lateral). The nail (horny) plate protrudes into these cracks with its edges. The nail plate is divided into root, body and edge. The root of the nail is called the back of the nail plate, lying in the back of the nail gap. Only a small part of the root protrudes from the posterior nail fissure (from under the posterior nail fold) in the form of a whitish area of a semilunar shape (lunula of the nail). The rest of the nail plate, located on the nail bed, makes up the body of the nail. The free end of the nail plate, protruding beyond the nail bed, is called the edge (protrusion) of the nail. The formation of the nail plate occurs due to the horny scales adjacent to each other, which contain hard keratin. The nail bed consists of epithelium and connective tissue. The epithelium of the nail bed is represented by the growth layer of the epidermis. The nail plate lying directly on it is its stratum corneum. The connective tissue of the bed contains a large number of fibers, some of which are parallel to the nail plate, and some are perpendicular to it. The latter reach the bone phalanx of the finger and connect to its periosteum. The connective tissue of the nail bed forms longitudinal folds in which blood vessels pass. The area of the epithelium of the nail bed, on which the root of the nail lies, is the place of its growth and is called the nail matrix. In the nail matrix, the process of reproduction and keratinization of cells is constantly taking place. The resulting horny scales are displaced into the nail (horny) plate, which as a result of this increases in size, i.e., the nail grows. The connective tissue of the nail matrix forms papillae, in which numerous blood vessels lie. Nail folds are skin folds. The growth layer of their epidermis passes into the epithelium of the nail bed, and the stratum corneum partially - into the nail plate, and partially moves over it from above (especially at its base), forming the so-called supraungual skin. skin glands There are three types of glands in the human skin - milk, sweat and sebaceous. The surface of the glandular epithelium of the sweat and sebaceous glands is approximately 600 times greater than the surface of the epidermis. These skin glands provide thermoregulation (about 20% of heat is given off by the body by evaporation of sweat), protection of the skin from damage (fatty lubrication protects the skin from drying out, as well as from maceration by water and moist air), excretion of some metabolic products from the body (urea, urinary acids, ammonia, etc.). Sweat glands are found in almost all areas of the skin. Their number reaches 2 - 2,5 million. The skin of the pads of the fingers and toes, palms and soles, axillary and inguinal folds is the richest in sweat glands. In these places for 1 cm2 more than 300 glands open on the surface of the skin, while in other parts of the skin there are 120-200 glands. The secretion of sweat glands (sweat) is a liquid with a low relative density, it contains 98% water and 2% solid residue. About 500 - 600 ml of sweat is released per day. Sweat glands can be subdivided into merocrine and apocrine glands. Apocrine glands are located only in certain places of the skin, for example, in the armpits, the anus, the skin of the forehead, and the labia majora. Apocrine glands develop during puberty and are somewhat larger. Their secret is richer in protein substances, which, when decomposed on the surface of the skin, give it a special, pungent smell. A variety of apocrine sweat glands are glands of the eyelids and glands that secrete earwax. Sweat glands have a simple tubular structure. They consist of a long excretory duct, running straight or slightly meandering, and of an equally long terminal section, twisted in the form of a ball. The diameter of the glomerulus is about 0,3 - 0,4 mm. The end sections are located in the deep parts of the reticular layer on its border with the subcutaneous fatty tissue, and the excretory ducts, having passed through both layers of the dermis and the epidermis, open on the surface of the skin, the so-called sweat pore. The excretory ducts of many apocrine glands do not form sweat pores, but flow together with the excretory ducts of the sebaceous glands into the hair funnels. The terminal sections of the merocrine sweat glands have a diameter of about 30 - 35 microns. They are lined with a single-layer epithelium, the cells of which, depending on the phase of secretion, can have a cubic or cylindrical shape. Drops of fat, glycogen granules and pigment grains are constantly found in the weakly basophilic cytoplasm of secretory cells. They usually contain highly active alkaline phosphatase. In addition to secretory cells, myoepithelial cells are located on the basement membrane of the terminal sections. By their contraction, they contribute to the secretion. The terminal sections of the apocrine glands are larger: their diameter reaches 150 - 200 microns. Secretory cells have an oxyphilic cytoplasm and do not have high alkaline phosphatase activity. In the process of secretion, the apical ends of the cells are destroyed and become part of the secret. The function of the apocrine sweat glands is associated with the function of the sweat glands - in the premenstrual and menstrual periods and during pregnancy, the secretion of the apocrine glands increases. The transition of the terminal section into the excretory duct is made abruptly. The wall of the excretory duct consists of a two-layer cubic epithelium, the cells of which are stained more intensely. Passing through the epidermis, the excretory duct acquires a corkscrew-like course. Here its wall is formed by flat cells. There are indications that when acetylcholine is introduced into the body, the metabolism of not only the cells of the terminal sections, but also the excretory ducts, increases. The sebaceous glands reach their greatest development during puberty. Unlike sweat glands, sebaceous glands are almost always associated with hair. Only where there is no hair (lips, nipples, etc.), they lie on their own. Most of the sebaceous glands are on the head, face and upper back. They are absent on the palms and soles. The secret of the sebaceous glands (sebum) serves as a fatty lubricant for the hair and epidermis of the skin. During the day, the human sebaceous glands secrete about 20 g of sebum. It softens the skin, gives it elasticity and facilitates the friction of the contacting surfaces of the skin, and also prevents the development of microorganisms on it. Unlike the sweat glands, the sebaceous glands are located more superficially - in the border sections of the papillary and reticular layers of the dermis. Near one hair root you can find 1 - 3 glands. The sebaceous glands in structure are simple alveolar with branched terminal sections. They secrete according to the holocrine type. The terminal sections, the diameter of which ranges from 0,2 to 2 mm, consist of two types of cells - poorly differentiated cells capable of mitotic division, and cells in different stages of fatty degeneration. The first type of cells forms the outer germ layer of the terminal section. Inside of it are larger cells, in the cytoplasm of which drops of fat appear. Gradually, the process of obesity intensifies, and at the same time the cells are shifted towards the excretory duct. Finally, obesity goes so far that there is cell death, which breaks down and forms the secretion of the gland. The excretory duct is short, opening into the hair funnel. Its wall consists of stratified squamous epithelium. Closer to the end section, the number of layers in the wall of the duct decreases, and it passes into the outer growth layer of the end section. Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu. << Back: Respiratory system >> Forward: Excretory system
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