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The skin must provide a covering for the rest of the tissues of the body. It must be strong enough to prevent fluids from leaking out, substances or microorganisms external to the body from getting in, but be flexible enough to provide for movement.

The skin is composed of an outer epidermis and a supporting dermis. The epidermis is comprised of squamous epithelial cells. These arise from a basal layer. The basal layer is constantly dividing to regenerate cells that are sloughed off. Thus, skin is a dynamic structure, which helps when an injury--such as a cut--takes place, because the injury will heal.

The cells derived from the basal layer begin to mature by developing a substance called keratin, which gives skin its toughness and resilience. With H&E staining, keratin gives cells a pink color. The surface of the skin is composed of a keratin layer derived from cells that have died. The thickness of the keratin layer is determined by the environmental stresses applied to the skin--the palms of the hands and soles of the feet have thicker keratin layers than the skin of the abdomen. A baby's skin is quite thin--with fewer squamous cells and less keratin--than an adult's skin.

Also seen in the epidermis are melanocytes. These cells contain the enzyme tyrosinase used to convert the amino acid tyrosine into the pigment called melanin. Melanocytic cell processes transport melanin granules into epidermal keratinocytes. Darker races have more melanin. The amount of melanin increases with sun exposure in lighter-skinned races--the "tanning" process.

Beneath the basal layer of the skin is a basement membrane, which serves to support the basal layer and serve as a barrier to fluids and substances that may move across. The basement membrane appears as a thin, dense, pink line.

Beneath the basement membrane is the dermis. The dermis is composed of collagen and elastic fibers made by fibroblasts. There is an upper papillary dermis with looser connective tissue with vessels and a lower denser reticular dermis. Collagen can be manufactured by fibroblasts in response to injury. However, the amount of collagen and elastic tissue diminishes with aging. In particular, ultraviolet light accelerates this process and damages the collagen and elastic fibers. Thus, older persons, particularly those who have been out in the sun for much of their lives, have more wrinkled skin.

Within the dermis are the skin adnexae, including eccrine sweat glands and hair follicles. The sweat glands help to cool the body by increasing production of sweat--mainly water with a variable amount of electrolytes (such as sodium chloride)--which evaporates from the skin surface. Hair follicles give rise to hair, the distribution of which is determined by sex and by age. Sebaceous glands also accompany hair follicles and produce an oily substance called sebum. The increased production of sebum in adolescence, particularly in males, can lead to blockage of the follicle and production of acne. A small bundle of smooth muscle called the arrector pili accompanies the hair follicle and the sebaceous gland, and all three compose the pilosebaceous unit.

Apocrine sweat glands are associated with the letter "A" and are found in axillae, areolae, and anal regions. Their gland lumens are large and they secrete into the hair canal. Their secretions produce characteristic odors when metabolized by skin bacteria, so they may function in sexual attraction.

Beneath the dermis is a layer of adipose tissue, or fat. Fat is composed of steatocytes (fat cells). The amount of adipose tissue beneath the skin is determined by uncontrollable factors such as sex and genetic factors, but modified greatly by dietary and exercise factors.