Anatomy, Physiology, and Disorders of Hair and Skin
Hair structure
The hair shaft has three layers: the cuticle (outer overlapping scales), the cortex (the thick middle layer holding melanin, where chemical and physical changes occur), and the medulla (the innermost core, sometimes absent in fine hair). The root sits in the follicle; its bulb covers the papilla, which supplies blood and nutrients. The sebaceous gland lubricates the hair and the arrector pili muscle causes goosebumps.
Growth cycles
- Anagen - active growth, roughly 2–6 years; about 85–90% of scalp hair is here.
- Catagen - transition, a few weeks.
- Telogen - resting and shedding, about 3 months; shedding 50–100 hairs daily is normal.
Hair types: vellus (fine, short, unpigmented) and terminal (long, coarse, pigmented).
Skin structure
The epidermis is the outer protective layer; the dermis holds collagen, elastin, nerves, glands, and follicles; the subcutaneous layer provides fat and insulation. Melanocytes in the epidermis produce pigment.
Disorders: recognize and sort
Non-contagious: alopecia areata (patchy loss), androgenic alopecia (pattern loss), canities (graying), trichoptilosis (split ends), fragilitas crinium (brittle hair), hypertrichosis (excess hair), hirsuties (excess hair in a pattern atypical for the sex). Contagious and referral-only: tinea (ringworm), pediculosis (lice), scabies. Never treat a contagious condition in the salon.
Traps to expect
The cortex does the work - questions often credit the cuticle with color change. Another trap: confusing hirsuties with hypertrichosis, or treating alopecia areata as contagious.
Worked example
A client has circular bald patches with smooth, clean edges and no scaling. Rule of thumb: smooth patches suggest alopecia areata (not contagious); scaly or broken hairs suggest tinea (refer). Either way, no chemical service until cleared.