Gartner_BRS Cell Biology & Histology, 9e

Chapter 12 Skin

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M

H

SG

B. A hair follicle is an invagination of the epidermis extending deep into the dermis. 1. The hair shaft (hair) is a long, slender filament positioned in the center of the hair follicle and extends above the surface of the epidermis. It consists of an inner medulla , cortex , and cuticle . At its deep end, it is continuous with the hair root . The cuticle of the hair is surrounded by the internal root sheath . 2. The hair root is the deepest expanded region of the hair follicle, containing a connective tissue projection, the dermal papilla , with a rich capillary network necessary for sustaining the folli cle. The hair root is separated from the dermal papilla by a basement membrane. a. The hair root contains keratinocytes that function as stem cells for hair shaft regeneration. In bald individuals, stem cells are present, but the mitotic and differentiation pathways are typically inhibited. b. The majority of the cells of the hair root comprise the matrix that eventually forms the me dulla (core of thick hairs) and the internal root sheath. (1) The internal root sheath is a cellular layer of the hair follicle that disappears above the level of the sebaceous gland duct. (2) The external root sheath forms the external most layer of the follicle and is a direct con tinuation of the strata basale and spinosum of the epidermis. (3) The glassy membrane , a thickened basement membrane, separates the hair follicle from the surrounding dermal sheath. c. Melanocytes are present in the matrix, lying on the basement membrane. The long dendrites of these cells extend between the keratinocytes of the cortex where melanin transfer occurs, and hair acquires various colors depending on the type and rate of melanin synthesis. FIGURE 12.6. Light micrograph of a portion of a hair follicle in thin skin. A hair shaft (H) is present within the follicle, and the surface of the skin is out of view at the bottom. Two sebaceous glands (SG) and an arrector pili muscle (M) are also observed. This muscle originates in the papillary layer of the dermis and passes obliquely to insert into the hair fol licle. When it contracts, it causes the hair to stand more upright, makes the surface of the skin dimple (causing “goose flesh”), and compresses the sebaceous gland so that it expresses sebum into the shaft of the hair follicle ( × 50).

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