Anatomy, Physiology, and Histology of Hair and Skin
What the exam tests
This domain tests hair structure, follicle anatomy, growth cycles, skin layers, and the glands and muscles associated with hair and skin.
Core rules
- The hair shaft has three layers: cuticle, cortex, and medulla. The cuticle is the outer protective layer; the cortex contains pigment and gives strength; the medulla is the central core and may be absent in fine hair.
- The follicle is the pocket in the skin. The bulb is the rounded base, and the papilla contains blood supply. The sebaceous gland adds oil, and the arrector pili muscle causes “goose bumps.”
- Growth cycles: anagen is active growth, catagen is transition, and telogen is resting/shedding.
- Skin layers: epidermis (outer), dermis (middle, with follicles, glands, nerves, and blood vessels), and subcutaneous layer (fat and insulation).
- The hair shaft itself has no nerve supply, which is why cutting hair does not hurt.
Worked example
A client asks why a haircut does not hurt even when the hair is pulled. Reason it out: the visible hair shaft is composed of keratinized cells and has no nerves. Sensation comes from the follicle and surrounding skin, so tension at the root can be felt, but cutting the shaft cannot stimulate a nerve.
Traps that cost marks
Confusing the follicle with the bulb; saying the shaft has blood supply; reversing anagen and telogen; and placing glands in the epidermis instead of the dermis.
Key points
- Shaft layers: cuticle, cortex, medulla.
- Follicle contains the bulb and papilla.
- Anagen grows, catagen transitions, telogen rests.
- Dermis holds follicles, glands, nerves, and vessels.
- The hair shaft has no nerves or blood supply.