New Concept of the Hair Cycle (Sebaceous Gland Hypothesis)
The cycle of alternating periods of rest and activity passed through by follicles of all types was first described in detail by Dry (1926) in the mouse standard model (Fig. 13.1a). During anagen (the active phase) the dermal papilla is almost enclosed by the cells of the matrix, which divide rapidly. Daughter cells move upward and differentiate to form the concentric layers of the hair itself and of the inner root sheath. A few epithelial cells are destined to become incorporated in the outer root sheath. Eventually, in the relatively short catagen phase, mitosis in the matrix ceases, and the base of the hair is keratinized to form the brush or club end, which then rises until it comes to rest just below the level of one-third of the anagen hair follicle. The follicle then passes into the resting phase (telogen), during which the inner root sheath of the lower portion of the follicle is no longer present and the club hair is retained in the follicle.
KeywordsSebaceous Gland Dermal Papilla Root Sheath Outer Root Sheath Dermal Papilla Cell
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