Anatomy and Physiology of Nails and Skin
Anatomy items on this exam are applied rather than rote: you are usually asked which structure produced a visible change, or why a service damaged the plate. Learn the parts of the nail unit, the function of each, and normal growth rates so you can reason to the answer.
The nail unit
- Nail plate: keratinized, roughly 100 layers, about 18% water; the plate itself has no nerves or blood supply, which is why filing it does not hurt until you reach tissue below.
- Nail matrix: sits under the proximal nail fold and produces the plate. It determines thickness and shape; the lunula is the visible front portion of the matrix. Matrix damage produces a permanent groove or distortion that grows out with the nail.
- Nail bed: dermis beneath the plate; its blood vessels give the healthy pink color. It is thin, so aggressive filing or drilling reaches it quickly.
- Eponychium is living tissue at the proximal fold; the cuticle is the dead, non-living seal on the plate. Also know the lateral folds, the hyponychium (seal beneath the free edge), and the mantle (the pocket that forms the proximal seal).
- Growth: fingernails about 3–3.5 mm per month (roughly 0.1 mm a day); toenails about 1.5 mm per month. Full replacement takes about 4–6 months on fingers and 9–12 months on toes. Growth speeds up in summer, in children, and on the dominant hand.
Skin
Epidermis, dermis, and subcutaneous layer; the stratum corneum is the barrier, and the nail plate is modified stratum corneum. Sebaceous glands are absent under the plate.
Worked example
A client reports a horizontal groove that has moved steadily toward the free edge over eight weeks. Because the groove travels with growth, it originated in the matrix during a period of interrupted growth (illness, trauma, aggressive prep) - not from surface filing. The correct response is to document, avoid further trauma, and refer if it recurs.
Traps that cost marks
Calling the cuticle "skin" or using the terms cuticle and eponychium interchangeably; claiming the plate grows from the nail bed; treating the lunula as a separate organ.