Hair growth cycle (anagen, catagen, telogen)

Every scalp follicle runs a repeating cycle: anagen, the active growth phase lasting years, in which the shaft lengthens continuously; catagen, a brief involution over a few weeks as the lower follicle regresses; and telogen, a resting phase of a few months that ends when the old shaft is shed — a step sometimes named separately as exogen — and a new anagen begins. On a healthy scalp the large majority of follicles are in anagen at any moment, and follicles cycle out of step with one another, which is why humans thin rather than moult.

The cycle is the clock behind almost every timeline in this field. Daily shedding of some tens of hairs is normal turnover, not disease. Treatments act on the clock — prolonging anagen, enlarging the hair it produces, or recruiting resting follicles back to work — and because a cycle turns over months, every honest therapy conversation carries the same warning: nothing visible happens quickly, and a few months is the earliest sensible checkpoint for judging any treatment.

Disorders map onto phases. Androgenetic alopecia progressively shortens anagen, so each generation of hair grows finer and briefer; telogen effluvium pushes a large cohort of follicles into telogen at once, producing diffuse shedding months after the trigger; and the shedding that follows a transplant is the same phase logic — implanted follicles drop their shafts, rest, and re-enter anagen over the following months.

That is why post-operative counselling is essentially cycle education: early shedding of transplanted hair is expected, not failure; regrowth begins as new anagen hairs surface months later; and results are judged at about a year, when enough cycles have restarted for the outcome to be real. Clinics that teach the cycle before surgery spend far less time managing panic after it — and patients who understand it can tell normal phase behaviour from a genuine problem.