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FIGURE 9-2 Layers of the epidermis. As specific cells in the basal layer replicate and differentiate, they move toward the skin surface. The major change in the cells is called ''keratinization" in which the cells become filled with a fibrous protein called keratin. Fully keratinized cells are constantly sloughed off at the skin surface. Other types of epidermal cells, such as melanocytes, have other functions. SOURCE: Reprinted from Sams and Lynch, 1990, with permission from Churchill Livingstone.

unprotected dermis. Such burns are quite painful because loss of an intact epidermis exposes cutaneous nerve endings to air, heat, cold, and other direct stimuli.

The epidermis contains several resident cell populations whose responses to certain stimuli can be protective or destructive. The Langerhans cell participates actively in recognition of and presentation of antigens. Lymphocytes respond to signals from Langerhans cells and other macrophages/monocytes and act in an appropriate fashion to antigens. Melanocytes, pigment-forming cells, protect the skin from harmful ultraviolet (UV) radiation.

The epidermis is composed of four biologically distinct layers of stratified squamous epithelium (Figure 9-2). The deepest layer, usually one cell thick, contains cells that continuously replicate and produce new cells at a rate sufficient to maintain an appropriate number of cells in the upper three layers of the epidermis. Based on location and function, this layer has been called the basal cell layer or germinative layer of the epidermis. Basal cells produce large quantities of the nucleic acids and nucleoproteins required in the process of cell division. Much

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