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skin, the flexible tissue (integument) enclosing the body of vertebrate animals. In humans and other mammals, the skin operates a complex organ of numerous structures (sometimes called the integumentary system) serving vital protective and metabolic functions. It contains two main layers of cells: a thin outer layer, the epidermis, and a thicker inner layer, the dermis. Along the internal surface of the epidermis, young cells continuously multiply, pushing the older cells outward. At the outer surface the older cells flatten and overlap to form a tough membrane and gradually shed as calluses or collections of dead skin. Horns, hoofs, hair (fur), feathers, and scales are evolutionary adaptations of the epidermis. Although the epidermis has no blood vessels, its deeper strata contain melanin, the pigment that gives color to the skin. The underlying dermis consists of connective tissue in which are embedded blood vessels, lymph channels, nerve endings, sweat glands, sebaceous glands, fat cells, hair follicles, and muscles. The nerve endings, called receptors, perform an important sensory function. They respond to various stimuli, including contact, heat, and cold. Response to cold activates the erector muscles, causing hair or fur to stand erect; fright also causes this reaction. From the outer surface of the dermis extend numerous projections (papillae) that fit into pits on the inner surface of the epidermis so that the two layers are firmly locked together. In humans, whorls on the fingers show where the epidermis falls between rows of papillae, making the patterns used in fingerprinting. The skin provides a barrier against invasion by outside organisms and protects underlying tissues and organs from abrasion and other injury, and its pigments shield the body from the dangerous ultraviolet rays in sunlight. It also waterproofs the body, preventing excessive loss or gain of bodily moisture. Human skin performs several functions that help maintain normal body temperature: its numerous sweat glands excrete waste products along with salt-laden moisture, the evaporation of which may account, in certain circumstances, for as much as 90% of the cooling of the body; its fat cells act as insulation against cold; and when the body overheats, the skin's extensive small blood vessels carry warm blood near the surface where it is cooled. The skin is lubricated by its own oil glands, which keep both the outside layer of the epidermis and the hair from drying to brittleness. Human skin has remarkable self-healing properties, particularly when only the epidermis is damaged. Even when the injury damages the dermis, healing may still be complete if the wounded area occurs in a part of the body with a rich blood supply. Deeper wounds, penetrating to the underlying tissue, heal by scar formation. Scar tissue lacks the infection-resisting and metabolic functions of healthy skin; hence, sufficiently extensive skin loss by widespread burns or wounds may cause death.
The entire outer surface of the body and the principal boundary between the external environment and the body's internal environment of cells and fluids. Skin serves as the primary barrier against the intrusion of foreign elements and organisms into the body, and also as a large and complex sense organ through which animals explore and learn about the external world. In addition, skin functions to maintain the homeostasis of the body's constituents, acting as a barrier to the loss of various ions and nutrients by diffusion. For terrestrial animals, it also serves as an effective barrier to water loss, without which most land animals would rapidly become desiccated and die.
The skin of humans and other mammals can be divided into two distinct regions, the epidermis and the dermis.
The epidermis is the outermost layer of the skin. It varies in thickness from 0.1 mm in most of the protected areas of the skin to approximately 1 mm in those regions exposed to considerable friction, such as the soles of the feet and palms of the hands. The epidermis consists of a great many horizontally oriented layers of cells. The outermost layer, the stratum corneum, consists of many layers of this packed cellular debris, forming an effective barrier to water loss from lower layers of the skin. The lowest levels of stratum germinativum constitute the portion of the skin that contains melanocytes, cells that produce the dark pigment melanin. Different levels of melanin secretion are responsible for the large range of pigmentation observed among humans.
The dermis plays a supportive and nutritive role for the epidermis. The epidermis has no blood supply of its own. However, nutrients and oxygen are apparently provided by diffusion from the blood supply of the underlying dermis. The average thickness of the dermis is 1–3 mm. It is in this layer that the sebaceous and sweat glands are located and in which the hair follicles originate. The products of all these sets of glands are derived from the rich blood supply of the dermis. Hair, sweat glands, and mammary glands (which are modified sweat glands) are skin inclusions unique to mammals. See Hair, Thermoregulation
the external covering of the body of animals and man. It protects the organism against external factors. In addition, its functions include sense of touch, metabolism (including gas exchange), excretion, and sometimes, thermoregulation. The skin prevents microorganisms and toxic substances from entering the body, protects the body from mechanical injuries, receives mechanical, thermal, and pain stimuli, determines the animal’s color, and excretes odorous or toxic substances or nutrients that are signals, defense mechanisms, or means of feeding offspring. In flying animals (for example, birds) or in animals that swim well, such as dolphins, the extremely high aerodynamic or hydrodynamic qualities are largely determined by the properties and active function of the skin and its derivatives.
The skin consists of an external (epithelial) layer and internal (connective-tissue) layers separated by a basal membrane and often forming numerous derivatives. The epithelial layer has a great variety of uni- or multi-cellular glands. The solid products of epithelial excretions may serve as skeletal and protective elements (for example, mollusk shells and the enamel on sharks’ scales). The products of the degeneration of some of the cytoplasm of the epithelial cells form a chitinous shell (the skeleton of arthropods) and an organ of chemical and mechanical protection—the cuticle of parasitic worms, which can resist the active digestive enzymes of the host’s intestine. Unlike invertebrates, which have a single layer of cells, vertebrates have a multilayered epithelium. In terrestrial vertebrates, all the cells of the external layers degenerate into horny tissue, forming a continuously renewed horny layer of epidermis, horny scales, hairs, feathers, and claws. The sloughing off of horny cells helps keep the skin clean. The horny layer in higher vertebrates and the chitinous shell in arthropods enables many of them to resist the drying action of the atmosphere on terrestrial animals. By retaining body heat, hairs and feathers make it possible for some mammals and birds to remain active even when the temperature is below 0°C.
In vertebrates and cephalopod mollusks the connective-tissue layer or dermis forms a network of collagenous and elastin fibers which make the skin strong and elastic. It forms bony and other types of scales in fish and cutaneous ossifications in reptiles, all of which function as armor.
An adult human being has 1.5–2 sq m of skin, whose thickness in different parts of the body varies from 0.5 to 4 mm. There are a number of layers in the epidermis: the basal and prickle-cell layers, which make up the germinative or malpighian layer, the granular layer, the hyaline layer, and the horny layer. Merkel cells are located in the lower layers of the epidermis. The sloughed-off cells of the horny layer are replaced by cells from the basal layer, which move to the surface. Thus, the various layers of the epidermis are stages in the conversion (differentiation) of basal-layer cells into horny cells. Because of differentiation, the cells lose their capacity for multiplication and for the accumulation of prokeratin fibrils—bundles of very fine (50–80 angstroms [A]) filaments of fibrous protein with a molecular weight of about 640,000. Later the accelerated synthesis of prokeratin causes the appearance of keratohyaline “granules” (accumulations of fibrous protein) in close contact with the fibrils.
The cells of the granular and hyaline layers, which contain a great deal of sulfurous amino acids, die after becoming almost solidly filled with keratin. Keratin consists of filaments about 75 A thick embedded in amorphous protein. As a result of oxidation, numerous SH groups of keratin molecules become locked into intermolecular S-S bridges. Keratin, which is characterized by considerable chemical stability, makes up about two-thirds of the weight of the cells in the horny layer.
The epidermal cells are completely renewed approximately every 20 days. In most mammals and in man, the epidermis forms sebaceous, sweat, and milk glands, as well as hair. The secretion of the sebaceous glands (sebum) lubricates the epidermis and hair, protecting them to some degree from getting wet. The sweat glands, which excrete several liters per day, have a role in thermoregulation. The evaporation of sweat from the surface of the skin absorbs excess heat and prevents the body from overheating. Some metabolic products are excreted in sweat (specifically urea, which is an important factor in kidney diseases).
The dermis consists of two layers: the papillary layer, which is next to the epidermis, and the reticular layer, which is deeper. The papillary layer forms papillae and crests that press into the epidermis, creating the markings on the skin. In addition to connective-tissue cells, the papillary layer contains a loose network of thin collagenous and elastin fibers, as well as smooth-muscle fibers. The latter help regulate the emission of heat. The reticular layer consists of dense connective tissue that forms a network of bundles of comparatively thick collagenous and elastin fibers that run parallel and at an oblique angle to the skin surface. Deep portions of the hair follicles and glands are embedded in the reticular layer. The epidermis and the dermis may contain pigment cells that protect the tissues against ultraviolet radiation and give the skin its color. (The pigment cells also account for the ability of some animals to change color readily.)
The blood vessels in the deep layers of the skin form an arterial network from which the vessels proceed to the surface layer. The lymphatic system consists of true lymphatic vessels located in the papillary layer of the dermis and connected to the subpapillary and subcutaneous lymphatic networks. The vast number of sensory nerve endings in the skin ensure a quick reaction to changing environmental stimuli. There are 100–200 pain receptors per sq cm of skin: 25 for touch alone, 12–15 for cold, and one to two for heat. The palm surface, the fingertips, and the tip of the nose are the most sensitive areas. Particularly on the fingertips, the skin is covered with complex markings that are highly specific to each individual and that do not change with age.
One of the main functions of human skin is thermoregulation. About 80 percent of all the heat emitted by the human body passes through the skin by convection, evaporation, and radiation. Virtually all the gases diffuse through the skin. However, skin respiration (entry of oxygen and elimination of carbon dioxide) constitutes only 1–1.5 percent of the total gas exchange. The skin also plays an important part in metabolism—including water-salt, carbohydrate, protein, and enzyme metabolism. The skin’s ability to absorb certain fat-soluble substances and volatile liquids makes possible the action of ointments and some externally administered drugs. Intimately associated with the vital activity of the entire organism, the skin is often affected by various organic and systemic functional disorders, which may be manifested in itching, burning sensations, redness, and rashes. Conversely, skin diseases or injuries may cause a general bodily reaction (for example, after-burn disease).
REFERENCESShmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonoch-nykh, 4th ed. Moscow, 1947.
Mnogotomnoe rukovodstvo po dermato-venerologii, vol. 2. Leningrad, 1961.
Gistologiia. Edited by V. G. Eliseev. Moscow, 1963.
Beklemishev, V. N. Osnovy sravnitel’noi anatomii bespozvonochnykh, 3rd ed., vol. 2. Moscow, 1964.
Pavlov, S. T. Kozhnye i venericheskie bolezni, 2nd ed. Leningrad, 1969.
Mercer, E. H. Keratin and Keratinization: An Essay in Molecular Biology. Oxford, 1961.
Montagna, W. The Structure and Function of Skin, 2nd ed. New York, 1962.
Biology of the Skin and Hair Growth. Edited by A. G. Lyne and B. F. Short. Sydney, 1965.
E. B. VSEVOLODOV and I. IA. SHAKHTMEISTER
skin(1) Regarding alternating current, see skin effect.
(2) A custom look and feel for a graphical interface (GUI). Operating systems and applications may have a built-in selection of skins or accept new ones from third parties. Also called "themes," skins include the background scenes, menu and button styles, colors, fonts, window borders and sometimes different arrangements and locations of the elements. See theme aware and app reskinning.
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(3) A covering for hardware. There are numerous vinyl coverings that stick onto cellphones, iPods, the backs of laptops and the sides of tower cases. They depict myriad subjects such as sports, high-tech and fantasy.
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