Hair growth cycle

The recurring sequence of growth, regression, and rest in a hair follicle. Its phases determine how long a hair grows and when it sheds.

Anagen: The active growth phase in which a hair follicle produces and lengthens a hair shaft. This phase accounts for most scalp hair growth and can last several years.
Hair shaft: The visible strand of hair, composed mainly of dead, keratinized cells produced by a follicle. Its emergence and loss make follicle cycling visible.
Telogen effluvium: A form of diffuse hair shedding caused by an unusually large number of follicles entering telogen together. It shows how synchronized shifts into rest can produce noticeable shedding.
Scalp hair: Hair growing on the scalp, with follicles that commonly have long growth phases. Its prolonged growth phase distinguishes scalp cycling from many other body sites.
Catagen: A brief transition phase in which hair production stops and the lower follicle shrinks. It marks the cycle’s shift from active growth toward follicle rest.
Hair matrix: A population of rapidly dividing cells near the base of a growing hair follicle. Matrix-cell activity builds the shaft during active growth.
Androgenetic alopecia: A patterned form of hair loss associated with inherited susceptibility and androgen effects on hair follicles. Affected follicles progressively produce shorter, finer hairs and spend less time growing.
Eyebrow: Short, coarse hair growing along the brow ridge above the eye. Eyebrow follicles have much shorter growth phases than scalp follicles.
Telogen: The resting phase in which a follicle retains its hair before shedding it and restarting growth. A follicle spends a substantial part of its cycle in this inactive state.
Hair bulge: A region of the outer hair follicle that houses stem cells used in hair and follicle regeneration. Its stem cells help rebuild the lower follicle as a new cycle begins.

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