Anatomy and Physiology of the Nail and Skin
Anatomy questions reward precise vocabulary: the exam asks you to name structures, describe growth, and separate a cosmetic condition from a disorder that requires referral.
The nail unit
- Nail plate - the hard keratin structure; three layers (dorsal, intermediate, ventral); no nerves or blood supply.
- Nail bed - living tissue under the plate, rich in capillaries, which is why bed injuries bleed.
- Matrix - where cells are produced; it extends under the proximal fold. The lunula is the visible portion of the matrix, not a separate organ. Matrix damage causes permanent ridges or grooves.
- Cuticle - the dead tissue seal on the plate; the eponychium is the living fold behind it. The hyponychium seals under the free edge; the perionychium is surrounding skin; the nail grooves are the side channels.
Growth
Fingernails grow about 3 mm (1/8 inch) per month and replace in four to six months. Toenails grow about 1 mm per month and take nine to twelve months. Growth slows with age, illness, poor circulation, and winter.
Conditions versus disorders
Onychomycosis (fungal), paronychia (bacterial), and onychocryptosis (ingrown) are referred, not treated. Onycholysis, leukonychia, koilonychia, onychorrhexis, Beau's lines, and pterygium can be described and documented but not diagnosed.
Worked example
A client has a green-black stain under an acrylic. It is likely Pseudomonas from moisture trapped under a lifted enhancement - a disorder. Remove the product, refer, disinfect, and do not reapply until cleared.
Traps
Calling the lunula "the matrix"; saying the plate has blood vessels; cutting live tissue; diagnosing instead of referring.