The nail matrix is the tissue under the proximal nail fold that produces the nail plate, forming about 90 percent of the plate from its germinal part, so any damage there shows as a mark in the growing nail.
Also known as: matrix unguis, nail root, germinal matrix, nail matrix cells
The nail matrix lies under the skin at the base of the nail, beneath the proximal nail fold. Its cells divide, flatten, fill with keratin and are pushed forward to become the nail plate. The germinal matrix produces about 90 percent of the plate. The visible end of the matrix is the lunula, the pale half moon (Medscape).
Because the plate is made here, everything that happens to the matrix appears later in the nail. A knock to the base of the nail can show as a white spot weeks afterwards. An illness with a high fever, chemotherapy or a zinc deficiency can pause growth and leave a horizontal groove, a Beau's line, that grows out over months (Cleveland Clinic). Permanent damage to the matrix, from a deep cut or a crushed finger, can leave a permanent split or ridge.
The cuticle seals the matrix against bacteria and fungi. Cutting the cuticle away, or pushing it back roughly, opens that seal. Pushing gently after softening and trimming only hangnails keeps it closed (Essie). Heavy filing at the base of the nail and rough removal of gel or acrylic also reach the matrix area.
Fingernails grow about 3 to 4 mm a month from the matrix, faster in summer and in people under 30. A full plate takes roughly 6 months to replace, so a mark made at the matrix today reaches the free edge in about half a year.