The integumentary system consists of the skin and associated structures, such as hair, nails, and glands. The skin is one of the body's main protections against toxins in the environment. Looking at this from a different point of view, the skin is also an important route of exposure for many toxins. The skin consists of two main layers, the dermis and the epidermis (Figure 9.1). The dermis, the inner layer, is made up primarily of connective tissue, plus sweat glands, hair follicles, nerve endings, blood vessels, and the small muscles that make your hair "stand up on end.'' The dermis contains a large amount of intracellular collagen fibers and elastic fibers. The elastic fibers give the skin the ability to stretch. The collagen fibers limit the stretching and give skin strength.

The epidermis, the outer layer, is about 1 mm thick. The epidermis does not have its own blood supply; it is supplied from capillaries in the dermis. The innermost part of the epidermis is the basal cell layer, a single layer of cells that replicate to replace lost

Figure 9.1 Layers of the skin. (From Van de Graaff and Rhees, 1997. © The McGraw-Hill Companies, Inc. Used with permission.)

epidermal cells. These new cells push up as other new cells are produced below. Once they are several cells away from the basal cell layer, they start producing large amounts of keratin, a tough fibrous protein, and the cells gradually die off and dehydrate. This process produces the outermost layer of the epidermis, called the stratum corneum. This layer consists of dead epidermis cells filled with keratin and sandwiched between phospholipid membranes. These cells are sloughed off continuously and regenerated by the epidermis. It takes about 15 to 30 days for a cell to go from the basal layer to the stratum corneum, and another 14 before it is shed.

The stratum corneum is resistant to water and other substances, although pores in the skin provide a route for substances such as drugs or toxins to enter. The solvent dimethyl sulfoxide (DMSO) easily crosses the skin and enters the circulation. Substances that otherwise do not easily cross the skin may do so if dissolved in DMSO. Melanin is the dark pigment in the skin of black people and of Caucasians with substantial exposure to sunlight (a tan). It is produced by cells in the basal layer called melanocytes. The melanin protects skin by absorbing ultraviolet light from the sun. Ultraviolet radiation from sunlight is a common cause of skin cancer.

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