Anatomy and Physiology of Skin, Hair, and Nails
Domain 3 - Anatomy and Physiology of Skin, Hair, and Nails
Questions here test structure and function: the layers of skin, the cells inside them, the life cycle of a hair, and the parts of the nail unit. You are expected to link structure to salon practice - why a cut cuticle invites infection, why a nail bruise grows out but a pigmented band does not.
Skin
- Epidermis, deep to superficial: basale, spinosum, granulosum, lucidum (palms and soles only), corneum. Keratinocytes dominate; melanocytes make melanin and shield DNA from UV; Langerhans cells handle immunity; Merkel cells sense touch.
- Dermis: collagen for strength, elastin for elasticity, fibroblasts, blood vessels, nerve endings, sweat and sebaceous glands.
- Hypodermis: fat and connective tissue for insulation and shock absorption.
- Functions: protection, sensation, thermoregulation, excretion, limited absorption, vitamin D synthesis. Cell turnover averages about 28 days.
Hair
- Shaft layers: cuticle (overlapping scales), cortex (bulk, keratin bundles, pigment), medulla (core, sometimes absent in fine hair).
- Follicle: bulb, matrix where cells divide, papilla supplying blood, sebaceous gland, arrector pili muscle.
- Cycle: anagen (years of active growth), catagen (weeks of transition), telogen (about three months' rest, then shedding). Scalp hair grows roughly 1 cm a month.
Nail unit
Plate, bed, matrix where the plate is formed, lunula, eponychium (cuticle), hyponychium, lateral folds and free edge. Fingernails grow about 3 mm a month, toenails about 1 mm; full replacement is roughly 4–6 months on hands and 12–18 months on feet.
Worked example
A client shows a brown vertical band on one fingernail, unchanged for six months. Reasoning: a bruise under the plate travels outward as the nail grows and eventually disappears; a band that stays put and extends onto the proximal fold is not trauma, so it must not simply be covered with polish - refer to a doctor.
Traps
- Calling the nail plate living tissue; it is keratinised and has no nerve supply.
- Believing cutting hair makes it grow thicker; cutting only changes the visible end.
- Cutting the eponychium, which breaks the seal and invites paronychia.