SKIN

in human anatomy, the covering, or integument, of the body's surface that both provides protection and receives sensory stimuli from the external environment. The skin consists of three layers of tissue: the epidermis, an outermost layer that contains the primary protective structure, the stratum corneum; the dermis, a fibrous layer that supports and strengthens the epidermis; and the subcutis, a subcutaneous layer of fat beneath the dermis that supplies nutrients to the other two layers and that cushions and insulates the body. The epidermis is made up of living and nonliving layers. Its innermost layers, the basal and spinous layers, are alive, and they produce cells, called keratinocytes, that form the dead outer layer, the stratum corneum, also called the horny layer. Keratinocytes contain the tough, insoluble group of proteins known as keratins that also make up hair and nails. The horny layer forms as keratinocytes, which become thin, hard, and dehydrated and migrate upward to the surface of the epidermis. The basal layer also contains melanocytesi.e., cells that produce the pigment melanin, which imparts colour to the skin and also protects it from the effects of ultraviolet radiation. Other cells found in the epidermis include Langerhans cells, which play a role in the body's immune defenses, and Merkel cells, which are involved in sensory reception. Structures such as hair follicles, nails, and sweat and sebaceous (oil-producing) glands are appendages that develop from the epidermis and extend into the dermis. The dermis takes up the greater part of human skin. It consists primarily of connective tissue through which an elaborate network of sensory nerves and blood vessels thread. The major component of dermal connective tissue is the extracellular fibrous protein collagen, which provides strength to the skin and resists deformation and tearing under stress. Fibres of collagen are organized loosely in the papillary layer of the dermis, which is adjacent to the epidermis. In the reticular layer, beneath the papillary layer, the collagen fibres are denser and more branched. Another fibrous protein, elastin, which is more flexible than collagen, is found in the dermis; fibres of elastin help the skin return to its original form after it has been stretched. Elastin fibres in the papillary layer also help to anchor the epidermis to the dermis. Surrounding these fibrous bundles is an extracellular gellike matrix called ground substance, which consists primarily of water, ions, and complex carbohydrates such as glycosaminoglycans that are often attached to proteins (proteoglycans). Ground substance helps to hold the cells of the tissue together and allows oxygen and nutrients to diffuse through the tissue to cells. Three types of cellsfibrocytes, histiocytes, and mastocytesare scattered throughout the dermis. Fibrocytes synthesize collagen, elastin, and ground substance. Histiocytes are a type of macrophagei.e., cells that dispose of cellular debris and invading microorganisms by engulfing and destroying them. Mastocytes, or mast cells, are located near blood vessels; they release substances that elicit such responses as fever, edema, and pain. The subcutaneous fat layer consists chiefly of fat cells (lipocytes) arranged into collagen-bound lobules. This fat serves as a nutritional storage depot for the entire body, and it cushions the skin and insulates the body against temperature extremes. See also integument. in human anatomy, the covering, or integument, of the body's surface that both provides protection and receives sensory stimuli from the external environment. The skin consists of three layers of tissue: the epidermis, an outermost layer that contains the primary protective structure, the stratum corneum; the dermis, a fibrous layer that supports and strengthens the epidermis; and the subcutis, a subcutaneous layer of fat beneath the dermis that supplies nutrients to the other two layers and that cushions and insulates the body. Additional reading Overviews are found in William Montagna and Paul F. Parakkal, The Structure and Function of Skin, 3rd ed. (1974), a textbook; Lowell A. Goldsmith (ed.), Biochemistry and Physiology of the Skin, 2 vol. (1983), articles by authorities in the field; and Arthur Rook et al. (eds.), Textbook of Dermatology, 4th ed., 3 vol. (1986), an outstanding work on normal and pathological human skin. Special aspects of the subject are discussed in Howard I. Maibach and Edward K. Boisits (eds.), Neonatal Skin: Structure and Function (1982); and R. Marks and P.A. Payne (eds.), Bioengineering and the Skin (1981). For current research in the field, see The Journal of Investigative Dermatology (monthly).