 Nail plate

The nail plate is the firm, keratinized material of a nail that lies over the nail bed and projects as the free edge. Its densely packed dead keratinocytes are produced through the growth process associated with the nail matrix. The plate protects the fingertip and supplies a backing surface during manipulation of small objects.^[ref]

The distinction between produced material and producing tissue makes a direct growth measurement intelligible. A mark embedded in the plate moves distally as new material is produced proximally; the exposed material does not lengthen by its own cells dividing. Measuring the current free edge alone mixes growth with clipping and wear.

An original mark-displacement example observes positions 2 and 8 mm from a fixed reference fold, sixty days apart. The mean advance is $(8-2)/60=0.10$ mm/day. If the reference landmark moves or the mark is worn away, that calculation no longer measures the same displacement. Repeated measurements should preserve both the mark and the anatomical reference.

A primary growth study used supplied nail files to mark nails in 22 healthy young adults and measured displacement again one to three months later.^[ref] It exemplifies following material rather than assuming that the free edge records all growth.

Nail-plate material age from growth marks needs more than dividing distance by a single population-average rate. If advance varies with time, a material element's displacement is the integral of its growth history. A clipping can also contain material formed over an interval. Measurements of chemicals in a clipping therefore require position, growth and external-contamination information before being interpreted as a precisely dated exposure record.

The plate, nail bed and nail matrix are complementary parts of one apparatus: transported material, supporting tissue and the region producing much of that material. Keeping them distinct avoids assigning vascular colour or proliferative activity to the exposed plate itself.