Hair follicle and the bulge stem cell niche
Hypodermis & appendages · Bulge stem cells Anagen 2-6 yr Re-epithelialisation
An epidermal downgrowth whose bulb sits in the deep dermis or hypodermis and whose cycling is directed by the mesenchymal dermal papilla. The bulge, at the arrector pili insertion, holds multipotent stem cells that regenerate the follicle, the sebaceous gland and, after wounding, the surface epidermis.
Traced from the start
- Stratum basale (germinativum)
- Lamina densa (type IV collagen basement membrane)Sublamina densa anchoring fibrils (type VII collagen)
- Papillary dermis
- Reticular dermis
- Hair follicle and the bulge stem cell niche
Detail
- Bulge
- Multipotent epithelial stem cells at the arrector pili insertion; the principal reservoir from which a partial-thickness wound or graft donor site re-epithelialises
- Cycle
- Anagen (growth) 2-6 years on the scalp and 85-90% of follicles at any time; catagen (involution) 2-3 weeks; telogen (rest) about 3 months and 10-15% of follicles
- Depth
- Terminal follicle bulbs extend to the deep reticular dermis and into the subcutis, so they survive burns that destroy the entire surface epidermis
- Absent from
- Palms, soles and volar digits, so wounds at those sites re-epithelialise only from eccrine ducts and the wound margin
When it goes wrong
Severe illness, childbirth, major surgery or crash dieting pushing follicles synchronously into telogen
Telogen effluvium: diffuse non-scarring shedding beginning 2-3 months after the trigger and recovering spontaneously
Dihydrotestosterone acting on genetically susceptible follicles via 5-alpha-reductase (androgenetic alopecia)
Progressive follicular miniaturisation with a receding frontal hairline and vertex loss in men, diffuse central thinning in women; treated with topical minoxidil and oral finasteride
Inflammation destroying the bulge (lichen planopilaris, discoid lupus, folliculitis decalvans)
Cicatricial alopecia: the stem cell niche is gone, so hair loss is permanent — unlike alopecia areata, where the follicle is preserved and regrowth remains possible
Practise this structure
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