Skin is an important and complex organ that performs various functions. It protects the body from external influences (mechanical, chemical, physical). Healthy and intact skin creates a reliable barrier to pathogens. She participates in the processes of respiration, excretion, thermoregulation; its role in metabolism and other vital processes is great.
The outer epidermal layer, or epidermis (I), is isolated in the skin. the skin itself – the dermis (II) and the subcutaneous tissue lying below it – the hypodermis (III).
The epidermis is formed by several dozen layers of epithelial cells lying one above the other. Its thickness is not the same everywhere, the thickest epidermis – up to 1.5 millimeters – has the skin of the soles, palms, and the thinnest – 0.03 millimeters – the skin of the eyelids.
The outer layer of the epidermis – the horny (1) – is mainly aged and keratinized epithelial cells. They are gradually desquamated and replaced by young cells. migrating here from the deep layers (2) of the epidermis. The timing of the complete renewal of the epidermis in different parts of the body is different: for example, on the sole this process lasts about a month. and on the elbow – ten days.
In the basal (3), the deepest layer of the epidermis, melanocytes are located – cells that produce the pigment melanin, the amount of which depends on the color of the skin: the less melanin, the lighter the skin, and vice versa. Melanocytes have the ability to increase the production of melanin under the influence of ultraviolet rays. It is thanks to this that tan comes in many different shades. There are people – they are called albinos – whose skin is devoid of melanin and therefore does not succumb to tanning.
The skin itself, or dermis, consists of densely interwoven connective tissue (collagen elastic and reticular) fibers and a few cells (4). The surface layer of the dermis adjacent to the epidermis and forming numerous protrusions is called the papillary layer (5). Especially numerous are the papillae in the skin of fingers, palms, feet. According to their location on the skin of these parts of the body, bumps are visible, various lines (they are called capillary), folding into a unique skin pattern that does not change throughout a person’s life.
Bunches of smooth muscle cells (6), attached to hair follicles (7), originate in the papillary layer. When a person is cold, these muscles contract, the hairs rise and the surface layers of the skin contract, forming “pimples”, while the heat transfer decreases slightly.
In the reticular layer (8) of the dermis following the papillary, there are many elastic fibers. These fibers give the skin elasticity. The strength of the skin is provided by other fibers of the same layer – collagen. Where the skin is subjected to constant pressure (on the feet, buttocks), a network formed by thick collagen fibers. – rough, wide-looped. And in places where the skin is constantly subjected to stretching (for example, on the face. In the joints), the collagen fibers are thin, and the network formed by them is finely looped.
Almost at the border of the dermis with subcutaneous tissue, hair roots (9) and sweat glands (10) lie. There are a lot of sweat glands in the skin of the palms of the hands, feet, in the axillary and inguinal region – up to 300 per square centimeter of the surface. Then, through the excretory ducts of the glands, potassium and sodium salts and other metabolic products are removed from the body. Sweating occurs constantly: it increases significantly with increasing ambient temperature, air dryness.
More superficially, mainly in the papillary dermis, are the sebaceous glands (11), the excretory ducts of which open into the hair funnels, as well as at the places where the hair exits to the skin surface. These glands, unlike sweat glands, are evenly distributed throughout the skin, they are not only in the skin of the palms, soles and red border of the lips. The sebum secreted by the glands (up to 20 grams per day) softens the skin, makes it more elastic.
The subcutaneous tissue, or hypoderm, located directly under the dermis, has a thickness of 2 to 10 millimeters. It is a layer of connective tissue that softens various mechanical effects on the skin and, in addition, serves as a kind of insulating pad. Subcutaneous tissue consists of collagen, elastic, reticular fibers, adipose cells, and lobules of adipose tissue (12). the amount of which depends on a number of factors – physique. lifestyle, nutrition. With a sedentary lifestyle, overeating, the thickness of subcutaneous fat can significantly increase, especially on the abdomen, hips, buttocks.