Anatomy and Physiology of the Nail and Skin
This domain covers the structures you touch and the biology you must not damage. Expect structures described in words, growth-rate questions, and "what happens if" scenarios about the matrix and the seal tissues.
The nail unit
- Nail plate - dead, translucent keratin; no nerves or blood vessels; the visible nail.
- Nail bed - pink tissue the plate rests on; packed with capillaries and nerves, which is why a lifted plate hurts.
- Matrix - the generator; extends a few millimeters under the proximal fold. Injury here causes permanent grooves or loss of the plate.
- Lunula - the whitish visible portion of the matrix.
- Eponychium - living skin at the base of the plate. The flaky tissue on the plate itself is the cuticle; it is a seal, not waste.
- Hyponychium - the seal under the free edge that keeps pathogens out.
- Lateral folds and grooves guide plate growth.
Growth facts
Average fingernail growth is about 0.1 mm per day, roughly 3 mm per month; a fingernail replaces in 3–6 months, a toenail in 9–12 months. Growth is faster in summer, in children, in pregnancy, and on the dominant hand; slower with age and poor circulation.
Skin
Epidermis (basal layer with melanocytes, then the keratinized stratum corneum), dermis (collagen, elastin, vessels, nerves, glands), subcutaneous layer. Beneath the nails sit the phalanges, with digital arteries and nerves running along each side.
Worked example. A client has a horizontal ridge across one fingernail two months after a door slammed on it. The ridge is not a disease and not a product reaction - it is a Beau's line, formed when the injured matrix paused growth and then resumed. Because it followed trauma on a single nail, no referral is needed unless it worsens; document it and avoid heavy pressure on the matrix.
Traps: calling the nail plate living tissue; confusing eponychium with cuticle; assuming toenails grow as fast as fingernails; trimming the hyponychium.