Anatomy and Physiology
Anatomy and Physiology tests your knowledge of the nail unit, skin, and underlying structures so you can recognize normal variation and avoid harming tissue. Questions frequently ask which structure produces the nail plate or why a ridge appeared.
The nail unit
The nail plate is made of keratinized cells; it has no nerves or blood supply. The nail bed lies beneath it and appears pink because of capillaries. The matrix, at the proximal end, produces the plate and determines its thickness and shape. The lunula is the visible portion of the matrix, not the whole matrix. The eponychium, often called the cuticle, is the living fold that seals the base; the thin dead tissue on the plate is the cuticle. The hyponychium seals under the free edge, and the lateral folds frame the sides.
Growth and skin
Fingernails grow about 0.1 mm per day, roughly 3 mm per month; toenails grow about half that rate. Growth is faster in summer, in younger people, and on the dominant hand. A fingernail replaces in about 4 to 6 months, a toenail in 9 to 12 months. Skin has three layers: epidermis, including the basal layer and melanocytes; dermis, with collagen, vessels, nerves, and glands; and subcutaneous tissue. Sudoriferous glands produce sweat; sebaceous glands produce sebum.
Worked example
A client has a horizontal ridge across one nail after a crush injury months ago. Reasoning: the matrix was temporarily damaged, so production paused or slowed; the ridge grew out with the plate. Do not file it flat or diagnose it. If the ridge is new, painful, or accompanied by discoloration, refer to a physician.
Traps
Confusing the lunula with the entire matrix, claiming the nail plate has blood vessels, cutting the eponychium deeply, and assuming toenails grow as fast as fingernails are frequent errors. Also remember that the nail plate is not living tissue, so trimming it is not painful; the surrounding tissue is.