fThe skin and its associated structures, hair, nails, and glands, are derived from surface ectoderm. Melanocytes, which give the skin its color, are derived from neural crest cells, which migrate into the epidermis. The production of new cells occurs in the germinative layer. After moving to the surface, cells are sloughed off in the horny layer (Fig. 18.1). The dermis, the deep layer of the skin, is derived from lateral plate mesoderm and from dermatomes of the somites.
Hairs develop from downgrowth of epidermal cells into the underlying dermis. By about 20 weeks, the fetus is covered by downy hair, lanugo hair, which is shed at the time of birth. Sebaceous glands, sweat glands, and mammary glands all develop from epidermal proliferations. Supernumerary nipples (polythelia) and breasts (polymastia) are relatively common (see Fig. 18.4).
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