= Nail matrix
The \b[nail matrix] is the proliferative growth region near the nail root that produces material incorporated into the <nail plate>. Cells divide and become keratinized as the resulting plate advances towards the fingertip. It is distinct from the supporting <nail bed> and from the free edge already exposed beyond the fingertip.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§5.2, Nails, proliferating basal cells at the root and Figure 5.13]{ref}
An original steady-volume model links production to motion. Let $J$ be net plate-material volume supplied per day, $w$ the plate width, $h$ its thickness and $v$ its advance speed. If material is approximately incompressible, the cross-section is uniform and losses before the free edge are negligible,
$$
J=whv,\qquad v=\frac{J}{wh}.
$$
For stipulated $J=0.30\,\mathrm{mm^3/day}$, $w=10$ mm and $h=0.30$ mm, the advance is 0.10 mm/day. Doubling width at the same production and thickness halves that modeled advance. These are geometric consequences of a chosen model, not universal differences between individuals.
<Matrix growth and plate-advection balance> also shows why a measured advance speed does not alone determine a cell-division rate. Cell size, keratinization, plate geometry and the spatial distribution of production enter the conversion from proliferation to material flow. A marker on the plate measures movement of produced material.
A primary study measured nail advance using marks in 22 healthy young adults; the reported mean fingernail rate was 3.47 mm/month and mean toenail rate 1.62 mm/month.<Growth rate of human fingernails and toenails in healthy American young adults by S. Yaemsiri and others>[Abstract, Methods and Results]{ref} Those measurements establish cohort growth observations, not direct counts of mitoses in the matrix.
A proposed comparison combines marked-plate displacement with measurements of plate width and thickness, keeping the observation interval and reference fold fixed. Interpreting changes in the inferred production rate requires checking the steady-geometry assumptions. The central anatomical distinction remains: the matrix supplies new material, while the mature plate records the movement and subsequent history of that material.
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