Anatomy and Physiology of the Hair and Scalp
This domain tests anatomy and physiology of the hair and scalp: shaft layers, follicle structures, growth cycles, and how these facts explain service choices.
Structures
- Hair shaft: cuticle (protective scales), cortex (bulk, melanin, bonds), medulla (central core, absent in fine hair).
- Follicle: bulb, papilla with blood supply, matrix where cells divide, sebaceous gland, arrector pili.
- Scalp layers: skin, connective tissue, aponeurosis, loose areolar tissue, pericranium.
Growth
- Anagen is growth, catagen is transition, telogen is rest, and exogen is shedding.
- Average scalp growth is about half an inch per month; shedding 50–100 hairs daily is normal.
- Sebum lubricates hair and scalp; hormones and age affect production.
Worked example
A client brings a shed hair with a small white bulb and asks if the root came out. The white bulb is a telogen club, not the follicle. The follicle remains in the scalp. This is a normal shed hair, so reassure the client and look for pattern, density changes, or scalp inflammation before recommending anything.
Traps
- Saying the medulla is always present.
- Claiming trimming increases growth rate.
- Confusing the bulb with the follicle.