Hair Follicle Anatomy
The hair follicle is a small organ extending into the dermis, made up of a bulb, a dermal papilla, root sheaths, a stem cell bulge, a sebaceous gland and the arrector pili muscle. On the scalp these follicles are clustered into follicular units of one to four hairs.
A follicle is an organ, not a hole. It is a tube of specialised skin extending down into the dermis, with its own blood supply, muscle, gland and stem cell reservoir. The visible hair is only the part that has already been built and pushed out.
| Structure | Where | What it does |
|---|---|---|
| Bulb | Deepest point | Where cells divide to build the shaft |
| Dermal papilla | Inside the bulb | Directs calibre and cycle length |
| Root sheaths | Around the growing shaft | Shape and guide the hair upward |
| Bulge | Mid-follicle | Stem cell reservoir that rebuilds each cycle |
| Sebaceous gland | Upper third | Produces the oil that coats the hair |
| Arrector pili | Bulge to skin surface | Small muscle that raises the hair |
The bulge is the reason follicles survive being moved. The regenerative cells sit above the bulb, not in it, which is why a follicle can be lifted out of the skin, spend hours outside the body, and still rebuild a full hair once it is placed. It is also why damage in the upper part of a graft is not a minor injury.
Follicles come out in groups. On the scalp they are clustered into follicular units of one to four hairs sharing a sebaceous gland and muscle. Those groups are the unit of surgery, which is why grafts and hairs are counted separately.
Related terms
For how these units are removed intact, see the FUE technique. For what a transplant actually moves, see what a hair transplant is.
