Maderson, Paul F. Department of Biology, Brooklyn College, Brooklyn, New York.
Last reviewed:January 2019
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- Growth and replacement
- Evolution and adaptive radiation
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Nonliving, specialized epidermal derivatives characteristic of modern mammals. Hair is a threadlike outgrowth of the epidermis of mammals. However, it is likely that hair was present in at least some therapsid reptiles. Hair consists of keratinized cells, tightly cemented together, which arise from the matrix at the base of a follicle (Fig. 1). A follicle is a tubular epidermal downgrowth that penetrates into the dermis and widens into a bulb (the hair root) at its deep end. The follicle, together with a lateral outgrowth called the sebaceous gland, forms the pilosebaceous system. Rapid cell production in the matrix, as well as differentiation in the regions immediately above, produces a hair shaft, which protrudes from the follicle mouth at the skin surface. See also: Epithelium; Mammalia; Sebaceous gland; Skin; Therapsida
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