Anatomy, Physiology, and Histology of Hair, Skin, and Nails
Hair
The hair shaft has cuticle (protective overlapping scales), cortex (bulk, melanin, disulfide bonds), and medulla (central core, often absent in fine hair). The follicle extends into the dermis; the bulb surrounds the papilla, which supplies blood and nutrients. Sebaceous glands attach to the follicle; arrector pili muscle causes “goose bumps.”
Skin
Epidermis is avascular and renews from the basal layer. Dermis contains collagen, elastin, blood vessels, nerves, and glands. Subcutaneous tissue stores fat and insulates. Know that exfoliation affects the epidermis, while massage affects circulation and muscle, not the nail matrix.
Nails
Nail plate rests on the nail bed. The matrix produces the plate; the lunula is the visible part of the matrix. The eponychium is the living cuticle; the hyponychium seals the free edge. The cuticle is dead tissue on the plate.
Worked example
A client has a pale, half-moon area at the nail base. That is the lunula, not the matrix itself. The matrix is beneath the proximal nail fold and produces the plate; damage there can cause permanent ridges. Reasoning: visible landmarks are not always the active tissue.
Traps
Saying the medulla is always present; confusing eponychium with cuticle; treating the nail plate as living tissue; and attributing hair color only to the cuticle.