Anagen growing, catagen regressing, telogen resting
Expansion
Structure: an invagination of epidermis, comprising the bulb with its dividing matrix cells and dermal papilla, the inner and outer root sheaths, and the shaft. Associated with it are the sebaceous gland and the arrector pili smooth muscle, which together form the pilosebaceous unit.
The cycle
- Anagen - growing, lasting 2 to 6 years on the scalp. About 85 to 90 per cent of follicles at any time
- Catagen - regressing, about 2 weeks
- Telogen - resting, about 3 months, ending in shedding. About 10 per cent
Because follicles cycle asynchronously, hair is normally shed a little at a time. Anything that synchronises them causes visible shedding.
Two types of hair loss follow directly
- Anagen effluvium: chemotherapy kills rapidly dividing matrix cells, so hair is lost within days to weeks
- Telogen effluvium: a stressor (childbirth, severe illness, surgery, crash dieting, thyroid disease, iron deficiency) pushes follicles into telogen together, so shedding occurs about 3 months later and recovers
Alopecia areata is autoimmune, with exclamation mark hairs and a lymphocytic infiltrate around the bulb.
Androgenetic alopecia results from dihydrotestosterone miniaturising follicles, which is why finasteride, a 5-alpha-reductase inhibitor, is a treatment.