Anatomy, Physiology, and Histology of Hair and Skin
The structure
Hair has two main parts: the hair root (below the scalp) and the hair shaft (above the scalp). The shaft has three layers: medulla (innermost), cortex (middle, contains pigment and strength), and cuticle (outermost, protective). The follicle is a tube in the skin; the bulb at its base contains the matrix and melanocytes.
Growth cycle
- Anagen - active growth phase.
- Catagen - transitional phase.
- Telogen - resting phase; shedding follows.
Skin layers
- Epidermis - outer protective layer.
- Dermis - contains follicles, glands, and vessels.
- Subcutaneous - fat layer.
Key glands and structures
- Sebaceous glands produce sebum, attached to follicles.
- Arrector pili muscles cause goosebumps.
- Nerves and blood vessels in the dermis supply sensation and nutrition.
Traps
- Confusing cortex (strength and pigment) with cuticle (protection).
- Forgetting that melanin is produced in the bulb by melanocytes.
- Mixing up the growth phases - anagen is growth, telogen is rest.
- Assuming hair is living tissue above the scalp - the shaft is not living.
Rule of thumb
Cuticle protects, cortex colors and strengthens, medulla is the core.
Worked example
A question asks where pigment is produced. Answer: melanocytes in the hair bulb at the base of the follicle, feeding pigment into the cortex as the hair grows.