 Nail (anatomy)

A nail is a specialized structure of the epidermis at the tip of a finger or toe. Its visible nail plate is a compact layer of dead keratinocytes rich in keratin. Living proliferating cells in the nail matrix produce the material that advances towards the free edge. The nail plate, its producing nail matrix and the supporting nail bed are therefore different parts of the apparatus.^[ref]

The firm nail plate protects the fingertip and supplies a backing surface when the finger manipulates a small object. The pink appearance mainly comes from the vascular tissue underneath, rather than blood vessels within the dead plate. Growth requires cell production at the base; the exposed distal material does not lengthen itself by dividing.^[ref]

A direct growth experiment follows a mark rather than repeatedly measuring the free edge, which can be clipped or worn away. S. Yaemsiri and colleagues studied 22 healthy young adults who used a supplied nail file to mark near the proximal nail fold. They recorded distance from the fold and repeated the measurement one to three months later. The observed mean rates were $3.47mm/month$ for fingernails and $1.62mm/month$ for toenails.^[ref] These are study averages, not universal rates for every person or digit.

For example, a mark moving from $1.0$ to $7.0\mathrm{mm}$ from the chosen fold over 60 days gives a mean advance $(7.0-1.0)/60=0.10mm/day$. This constructed calculation measures displacement over that interval. Interpreting a chemical measurement from a clipping as a dated exposure record requires more: the person's growth history, the clipping's position and possible external contamination all matter. A single current growth-rate estimate does not establish an exact date for every part of the nail plate.