A tube in the dermis whose base bulb sits over a knot of blood vessels called the dermal papilla
Hair follicle — the papilla feeds the matrix cells.
Anatomy · Glands & the immune system
Hair follicle
A tiny organ that grows a hair
Integumentary system · dermis
A tube-shaped pit sunk from the surface epidermis down into the dermis (and, for large hairs, into the fat beneath), in which a hair is made. At its base the bulb wraps around a knot of connective tissue, the dermal papilla, which carries the blood supply; the matrix cells around it divide rapidly and push the growing hair upward. A sebaceous gland opens into the follicle's upper part, and a tiny smooth muscle, the arrector pili, attaches to it and raises the hair to give goose bumps.
How to spot them
- ▸Slanting tube in section, with a bulb-shaped swelling at its base holding a small pink papilla
- ▸Dark hair shaft inside layered sheaths, with a sebaceous gland budding from the side
- ▸Slender band of smooth muscle running from the follicle to the dermal surface above
- ▸Occurs everywhere on the body except the palms, soles and lips
- ▸Stem cells live in a bulge on the outer sheath where the muscle attaches
What to know
The tiny smooth muscle attached to its side that pulls the hair upright when the sympathetic nerves fire
Hair follicle — the arrector pili produces goose bumps.
A three-stage life cycle of anagen (growth, years), catagen (transition, weeks) and telogen (rest, months)
Hair follicle — the cycle governs shedding; about 50 hairs a day are lost.
A gland that opens into its neck and oils the hair with sebum, forming a 'pilosebaceous unit'
Hair follicle — the sebaceous gland is part of the unit.
A reservoir of stem cells in the 'bulge' region regenerates it and can help repair the surface
Hair follicle — bulge stem cells.
The curved shape of this tube in the scalp determines whether hair emerges straight or curly
Hair follicle — the more curved the follicle, the curlier the hair.
About 100,000 of these sit in the scalp; none are found on palms or soles
Hair follicle — the palms, soles and lips are hairless (glabrous).
Key ideas
- Matrix cells at the base of the follicle are among the fastest-dividing cells of the body. They differentiate into the hair's medulla, cortex and cuticle and into the inner root sheath, becoming keratinised and dying as they are pushed upward.
- Melanocytes in the bulb pass pigment to the growing hair; when they run out, hair turns white or grey.
Quick-fire facts
- •Hair on the scalp grows about 0.3 mm a day (roughly 1 cm a month) during the anagen phase
- •Scalp anagen phase lasts years, catagen about 2–3 weeks, telogen about 2–4 months (OpenStax)
- •The follicle has an outer root sheath continuous with the epidermis, and an inner root sheath that moulds the hair
- •The hair shaft has three zones: an inner medulla (absent in fine hairs), a cortex and an outer cuticle of overlapping scales
- •Humans have several million follicles in all, about 100,000 on the scalp, the number varying with hair colour
- •Alopecia areata is an autoimmune attack on the follicles; male-pattern baldness is driven by dihydrotestosterone shrinking scalp follicles
- •Sebaceous glands, sweat glands and the arrector pili muscle are its companions in the dermis
- •Hair follicles are one of the few organs that regenerate repeatedly through life
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