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D. Merkel's cells

D. Merkel's cells: Scattered in the stratum basale, these cells are most numerous in thick skin. They resemble basal keratinocytes but have a clearer cytoplasm containing many small dense granules. Free nerve endings form a disklike expansion (Merkel's disk) that covers the basal surface of each Merkel's cell. This arrangement suggest that the cells function as sensory mechanoreceptors, but other evidence suggests that they may have DNES-related functions

C. Langerhans' cells

C. Langerhans' cells: These star-shaped cells lack tonofilaments and occur mainly in the stratum spinosum (400-1000 cells/mm2 of skin surface). They stain selectively with gold chloride and contain numerous rodlike or racket-shaped cytoplasmic granules (Birbeck's granules). They are thought to be antigen-presenting cells that process and present to the lymphocytes any antigenic material that penetrates the skin's surface. Of mesodermal origin, they arise in bone marrow and may belong to the mononuclear phagocyte system. Langerhans' cells also occur in oral and vaginal epithelia as well as in the thymus.

B. Pigmentation System

B. Pigmentation System: Skin color is conferred mainly by the pigments melanin and carotene, the thickness of the epidermis, the number of dermal blood vessels, and the color of the blood in those vessels.

1. Melanins contribute to skin, eye, and hair color. Synthesized by melanocytes, they include the dark brown pigment eumelanin, found in the epidermis, iris, and brown and black hair; and the cysteine-rich pigment pheomelanin, found in red hair. 2. Melanocytes derive from the neural crest and migrate into the epidermis during em bryogenesis. Although they are scattered among the keratinocytes of the stratum basale, they are not attached to them by desmosomes. They have round cell bodies, central nuclei, and long cytoplasmic processes that pass between the cells of the strata basale and spinosum and terminate in small indentations on the keratinocyte surfaces. Melanocytes make up 10-25% of this layer's cells but do not participate in keratinization.