Dermatology => Skin
Skin, in anatomy, protective organ covering the external surface of the body and merging, without break, at the various body openings, with the mucous membranes of the alimentary and other canals. The skin forms a protective barrier against the action of physical, chemical, and bacterial agents on the deeper tissues and contains the special end organs for the various sensations commonly grouped as the sense of touch. Through the activity of its sweat glands and blood vessels, it is important in maintaining body temperature. One square inch (6.5 sq cm) of skin contains up to 4.5 m (15 ft) of blood vessels, which have as one of their functions the regulation of body temperature.
When the body temperature rises, radiant energy, or heat, is lost due to vascular dilation and increased blood flow to the skin surface. When the temperature is low, blood vessels constrict to reduce blood flow and subsequent heat loss through the skin. Each square inch of skin also contains hundreds of sweat glands that are controlled by a heat regulation center in the brain. These glands secrete moisture, which evaporates, cools the body surface, and helps maintain normal body temperature. In this capacity the skin acts as an excretory organ. The skin is elastic, and except for a few areas such as the palms, soles, and ears, is loosely attached to the underlying tissues. The color of the skin varies normally with the amount of pigment deposited in the skin cells as governed by heredity and by exposure to sunlight. The color also varies in disease because of difference in pigment, as in Addison's disease, or because of pigmented substances carried to it by the blood as in jaundice. In certain places the outer layers of the skin are modified to produce the hair and nails. The skin varies in thickness from 0.5 mm (0.02 in) on the eyelids to 4 mm (0.17 in) or more on the palms and soles.
In structure the skin is composed of two distinct layers. The outer layer, called the epidermis or cuticle, is several cells thick and has an external, horny layer of dead cells that is constantly shed from the surface and replaced from below by a basal layer of cells, the stratum germinativum. The inner layer, called the corium or dermis, is composed of a network of collagen and elastic fibers, blood vessels, nerves, fat lobules, and the bases of hair follicles and sweat glands. The interface between the epidermis and dermis is extremely irregular and consists of a succession of papillae, or fingerlike projections, which are smallest where the skin is thin and longest in the skin of the palms and soles. The papillae of the palms and soles are associated with elevations of the epidermis which produce ridges that are the basis for fingerprint identification. Each papilla contains either a capillary loop of blood vessels or a specialized nerve ending. The vascular loops, supplying nutrients to the epidermis, outnumber the neural papillae by about four to one.
Sweat, or sudoriferous, glands are found on every part of the body. They are numerous on the palms and soles but relatively sparse on the skin of the back. Each gland consists of coiled tubules that are situated in the subcutaneous tissue and a duct that extends through the dermis and forms a convoluted spiral through the epidermis. Sebaceous glands, saclike glands that secrete the sebum that lubricates and softens the skin, open into the hair follicles a short distance below the surface of the epidermis.
The skin is prone to diseases from external as well as internal causes. Inflammation of the skin, or dermatitis, may often result from exposure to physically or chemically irritating substances in industry, from contact with such vegetable poisons as the toxin of poison ivy, or from sunburn, overexposure to the ultraviolet rays of the sun. Infection of the skin with pyogenic streptococci gives rise to impetigo and erysipelas, and infections of the skin spread throughout the body, as in syphilis, smallpox, and tuberculosis; general systemic disorders may produce skin symptoms, as in measles, scarlet fever, and chicken pox. Foreign proteins, or proteins to which the body is unduly sensitive, may affect the skin by producing hives, or wheals, whether the protein reaches the skin through the bloodstream or whether it is applied directly to the skin. Patients are often tested by placing a small amount of the protein on a small scratch in the skin; sensitivity is indicated by the appearance of a wheal . Eczema, formerly considered the most common skin disease, is now regarded as a symptom of any of a variety of conditions, including external local irritations, disorders of the blood, and allergy. Other skin diseases include tumors, sebaceous cysts (wens), ulcers and pigmentations that are congenital or are caused by disorders of the internal secretions, and melanoma .
Destruction of extensive areas of skin is sometimes caused by injury, as in the case of burns, by surgical removal, or by disease, as in large ulcers. Regeneration of the skin over such denuded areas takes place naturally by proliferation of cells from the surrounding margins of healthy skin and the underlying skin appendages. The formation of scar tissue, however, prevents the regrowth of skin over the denuded area and may cause disability of the part by forming contractures or adhesions. Complete coverage of the damaged area is facilitated by the use of skin grafts, sections of skin of either full thickness or partial thickness depending on the indications, cut from another part of the body (the donor site) and applied to the raw surface (the recipient site), to which they adhere readily. In successful cases, the graft is nourished at first by the serum oozing from the damaged tissue, and later by the proliferation of capillaries from the tissue into the graft. Eventually it merges with the surrounding skin to cover the entire area.
In general, permanent grafts can be made only from skin transplanted from the body of the same individual, called autografts, or from an identical twin. With the exception of transplants from identical twins, graft transplants are usually sloughed in about three weeks.
Artificial skin grafts have also been developed to serve as temporary protection during healing. They consist of a dermal layer of animal protein fibers and an epidermal layer of a silicon plastic, which is known as Silastic. In addition, skin may be cloned; however, the resulting skin lacks flexibility for growth.