Sebaceous glands secrete sebum that coats the hair and skin surface
Sebaceous glands develop as outgrowths of the external root sheath of the hair follicle, usually producing several glands per follicle (see Fig. 14.14). The oily substance produced in the gland, sebum, is the product of holocrine secretion. The entire cell produces and becomes filled with the fatty product while it simultaneously undergoes programmed cell death (apoptosis) as the product fills the cell. Ultimately, both the secretory product and cell debris are discharged from the gland as sebum into the in-fundibulum of a hair follicle, which with the short duct of the sebaceous gland forms the pilosebaceous canal. New cells are produced by mitosis of the basal cells at the periphery of the gland, and the cells of the gland remain linked to one another by desmosomes. The basal lamina of these cells is continuous with that of the epidermis and the hair follicle. The process of sebum production from the time of basal cell mitosis to the secretion of the sebum takes about 8 days.
Eccrine sweat glands are independent structures, not associated with the hair follicle that arises as a downgrowth from the fetal epidermis. Each eccrine gland is arranged as a blind-ended, simple, coiled tubular structure. It consists of two segments: a secretory segment located deep in the dermis or in the upper part of the hypodermis and a di-
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