Understanding how hair is structured can make it easier to understand shedding, thinning and the limits of different hair-loss treatments. A visible hair fibre is only one part of a larger follicular unit that sits within the skin.
Hair shaft and follicle
The hair shaft is the part that extends above the skin. It is made largely of keratin and is not living tissue. The follicle is the skin structure from which the hair grows. At its base, rapidly dividing matrix cells contribute to formation of the hair shaft, while the dermal papilla plays an important role in signalling and growth regulation.
Scalp follicles can naturally occur in follicular units containing one or more hairs. This is relevant to hair-transplant planning because surgical grafts are based on naturally occurring follicular units rather than artificial bundles of hair.
Main structures around a follicle
- Hair bulb: the enlarged lower portion of the growing follicle.
- Dermal papilla: a small structure at the base of the follicle involved in growth signalling.
- Matrix: rapidly dividing cells that contribute to formation of the hair shaft.
- Inner and outer root sheaths: layers surrounding and supporting the developing hair within the follicle.
- Hair shaft: the keratinised fibre that emerges above the skin.
The hair growth cycle
Individual scalp hairs cycle independently through several phases rather than all growing or shedding at the same time.
- Anagen: the active growth phase, which can last for years on the scalp.
- Catagen: a short transition phase during which active growth slows and the follicle changes.
- Telogen: a resting phase before the old hair is eventually shed and a new cycle begins.
- Exogen: the shedding stage in which the old hair fibre is released.
Why this matters in hair loss
Different conditions affect different parts of the follicle or growth cycle. For example, some disorders increase the proportion of hairs entering a resting or shedding phase, while androgenetic hair loss gradually miniaturises susceptible follicles. That is why persistent or unusual shedding should be assessed by cause rather than treated with a single generic solution.
For more context, see the hair loss information. Hair transplantation is relevant only for selected patients with suitable donor follicles and does not correct every disorder of the hair-growth cycle.