= Nail plate
The \b[nail plate] is the firm, keratinized material of a <nail (anatomy)> 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.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§5.2, Nails and Figure 5.13]{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.<Growth rate of human fingernails and toenails in healthy American young adults by S. Yaemsiri and others>[Abstract, Methods]{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.
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