Anatomy and Physiology of the Skin and Hair
Anatomy and Physiology of the Skin and Hair
This domain tests the structure and function of skin and hair, and your ability to link that anatomy to what a treatment can and cannot achieve.
Skin structure
- Epidermis - avascular, built from keratinocytes. Layers from base to surface: basale, spinosum, granulosum, (lucidum on palms and soles only) and corneum. Turnover is roughly 28 days and slows with age.
- Epidermal cells - melanocytes in the basal layer produce melanin for UV protection; Langerhans cells are immune sentinels; Merkel cells detect touch.
- Dermis - papillary and reticular layers containing collagen (mostly type I), elastin, glycosaminoglycans and hyaluronic acid, made by fibroblasts, plus blood vessels, nerves and glands.
- Hypodermis - adipose tissue for insulation, cushioning and energy storage.
- Functions - protection, sensation, thermoregulation, vitamin D synthesis, excretion and limited absorption. Absorption is the basis of topical product penetration.
Hair structure and growth
The shaft has a cuticle of overlapping scales, a cortex of keratin and melanin, and a medulla that fine hair may lack. The follicle contains the bulb and papilla (blood supply), a sebaceous gland and the arrector pili muscle. Growth cycles through anagen (2–7 years on the scalp, about 1 cm per month), catagen (2–3 weeks), telogen (about 3 months) and exogen. Androgens convert vellus hair to terminal hair at puberty.
Worked example
A client asks why a glycolic product stings more on the cheeks than the forehead. The stratum corneum is thinner on the cheek and nerve endings are denser, so both penetration and sensation are greater. You reduce contact time and strength on that area rather than assuming the product is faulty.
Traps that cost marks
Saying the epidermis contains blood vessels; describing hair as growing continuously rather than in cycles; confusing sebaceous glands (sebum) with eccrine and apocrine sweat glands; placing the stratum lucidum on the whole body.