Skin (hereinafter – the “S.”) – is the outer covering of the body of animals and humans, exercising functions of defense against external influences, as well as touch, metabolism (including gas exchange), excretion, sometimes thermoregulation. S. prevents penetration into the internal environment of microorganisms, toxins, mechanical damage, takes mechanical, thermal and painful effects, defines animal coloring, highlights odorous, toxic or nutritional substances serving signals in security or rearing of offspring.
The skin consists of an outer (epithelial) and internal (connective tissue) layers separated by a basal membrane and often forming numerous derivatives (see Fig. 1, 2, 3).
Fig. 1. The structure of the skin insect (scheme): 1 – epicuticle; 2 – exocuticle; 3 – epidermis; 4 – trihogennaya (hair) cell; 5 – connective cells; 6 – basement membrane of the epidermis; 7 – hair; 8 – endocuticle.
Fig. 2. Structure of the skin bony fish (scheme): 1 – rich glandular cells of the epidermis; 2 – bony scales; 3 – connective tissue layer.
Fig. 3. Structure of the skin of a mammal (the scheme): 1 – hair; 2 – the horny layer of the epidermis; 3 – other layers of the epidermis; 4 – papillae of the outer layer of the dermis, jutting out into the epidermis; 5 – sebaceous glands; 6 – sweat glands; 7 – fatty subcutaneous tissue; 8 – reticular layer of the dermis.
Epithelial layer of a large diversity of single-or multi-cellular glands. Solid products of epithelial excretions may serve as skeletal and protective elements (shells of mollusks, enamel scales of sharks). Products of degeneration of the cytoplasm of epithelial cells form a chitinous shell – the skeleton of arthropods and body chemical and mechanical protection – the cuticle of parasitic worms to counter the activity of digestive enzymes of the host intestine. Unlike invertebrates, having a layer of cells in the epithelium, K., multilayered epithelium in vertebrates, and at the vertebrate cells of the outer layers are completely horny degenerate and form a constantly renewed horny layer of the epidermis (see Fig. 4), horny scales, hair, feathers, claws and more. Desquamation of horny cells helps to cleanse the skin of dirt.
Fig. 4. The structure of mammalian epidermis (the scheme): 1 – basal layer; 2 – layer ribbed; 3 – granular layer; 4 – shiny layer; 5 – horny layer; 6 – basement membrane; 7 – fiber papillary dermis; Н – the “grain” keratohyalin; Ф – fibrils; Я – the nucleus of cells; O – thorny process of cell layer.
Stratum corneum of higher vertebrates and arthropods chitinous shell allows many of them to withstand the withering work atmosphere in a terrestrial lifestyle. Hair and feathers, keeping body heat, contribute to some mammals and birds maintain active life even at t temperatures below 0° C. Connective tissue layer, or dermis, forming a network of collagen and elastin fibers that give skin strength and elasticity (in vertebrates, cephalopods), and other bone and scales (fish) and dermal ossification (reptiles), playing the role of armor.
In the adult area of the skin is 1.5-2 m2. The thickness of the various parts of the body varies from 0.5 to 4 mm. In the epithelial layer (epidermis) S. distinguish layers: basal, ribbed (comprising germ or Malpighian, layer), grainy, shiny and horny, and in the lower layers of the epidermis are sensitive. Exfoliated cells of the horny layer are replaced by shifts to the surface cells of the basal layer. So called layers – the stage of conversion (differentiation) of cells in the basal layer of the horn.
Differentiation is accompanied by a loss of the ability of cells to multiply and accumulation of fibrils prokeratina, that is the finest beams (50 – 80) filaments of fibrous protein with a molecular weight of about 640 000. Next, in close contact with the fibrils appear “grain” keratohyalin, ie clusters of fibrous protein. Their appearance is associated with accelerated synthesis prokeratina. Granular cells and brilliant layers include a large amount of sulfur-containing amino acids, and almost entirely filled with keratin and die off. Keratin filaments represented about 75 thick, immersed in an amorphous protein. Numerous SH-groups of molecules of keratin oxidation closed in intermolecular S – S-moctiki. Keratin is about 2/3 the weight of the stratum corneum cells and is characterized by high chemical resistance.
Complete upgrade of the epidermal cells is about 20 days. In most mammals, and humans epidermis forms sebaceous, sweat glands, mammary glands and hair. Secretion of sebaceous glands (sebum) lubricates the epidermis and hair, making them water-repellent. Sweat glands are involved in thermoregulation, allocating up to several liters of sweat per day, the evaporation from the surface of which K. removes excess heat and prevents overheating of the body. Since then leave the body and part of the products of metabolism, especially urea, which is of particular importance in renal disease.
The dermis distinguish two layers: papillary, adjacent to the epidermis, and the mesh and deeper. The first images of the different forms of papillae and ridges jutting into the epidermis and determine the design surface of the skin. Papillary layer, along with connective tissue cells contain a loose network of thin collagen and elastin fibers and smooth muscle fibers. Last involved in the regulation of heat transfer. Reticular layer of dense connective tissue before forming a network of relatively thick bundles of collagen and elastin fibers, oriented parallel to and at oblique angles to the surface of S. mesh layer immersed deep sections of hair follicles and glands.
The epidermis and dermis may contain pigment cells that protect the fabric from UV rays and give skin a different, some animals easily changed, painting. S. blood vessels in the deeper layers form a skin arterial network, from which depart the vessels in the surface layer. The lymphatic system consists of true lymphatic vessels in the papillary dermis associated with papillar and subcutaneous lymphatic network.
A huge number of sensory nerve endings located in S., provides quick response to the changing external environment. On 1 cm2 of skin surface pain points is 100 – 200, tactile sensitivity – 25, cold – 12 – 15 and the heat – 1-2. The greatest sensitivity is S. palmar surface, the fingertips and the tip of the nose. S. surface, especially at the fingertips, covered a complex pattern, which is strictly individual and age does not change (on this property based skin forensic science – fingerprinting).
One of the main functions of human skin – thermoregulation. About 80% of the heat transfer occurs through S. convection, evaporation and radiation. S. diffuse through almost all gases. However, skin respiration (oxygen and removing carbon dioxide) is only 1 – 1.5% of the total gas exchange.
Plays a significant role in skin metabolism – water-salt, carbohydrate, protein, enzyme, etc. Through S. absorbed some substances that are soluble in fats and volatile liquids, based on what the effect of certain drugs used topically, and ointments. S. inseparably linked with the life of the whole body – the various dysfunctions of organs and systems often affect the state S. (itching, burning, redness, rashes and other reactions), and, conversely, the disease S. cause a reaction of the body (for example, burn disease ).