Clinicians used to observing the slow rate of clearance of diseased or damaged nails are
apt to view the nail apparatus as a rather inert structure, although it is in fact the centre of
marked kinetic and biochemical activity.

Cell kinetics
Unlike the hair matrix, which undergoes a resting or quiescent (telogen) phase every few
years, the nail matrix germinative layers
continue to undertake DNA synthesis, to divide and to differentiate throughout life, akin
to the epidermis in this respect. Exactly which parts of the nail apparatus contribute to the
nail plate has been debated; it is now usually accepted that the three-layer nail plate is
produced from the proximal matrix, the distal matrix and the nail bed (sterile ventral
matrix).

Why the nail grows flat, rather than as a heaped-up keratinous mass, has generated
much thought and discussion. Several factors probably combine to produce a relatively
flat nail plate; the orientation of the matrix rete pegs and papillae, the direction of cell
differentiation, and the fact that since keratinization takes place within the confines of the
nail base, limited by the proximal nail fold dorsally and the terminal phalanx ventrally,
the differentiating cells can only move distally and form a flat structure—by the time they
leave the confines of the proximal nail fold all the cells are dead, keratinized and
hardened.

Linear nail growth
Many studies have investigated the linear growth rates of the nail plate in health and
disease; their findings are summarized in Tables 1.1 and 1.2. Finger nails grow
approximately 1 cm every 3 months and toe nails at half this rate.

Table 1.1 Physiological and environmental factors affecting the rate of linear
nail growth

Faster growth
Day-time
Pregnancy
Minor trauma/nail biting
Right-hand nails
Youth, increasing age
Fingers
Summer
Middle, ring and index fingers
Male (?)

Slower growth
Night-time
First day of life

Left-hand nails
Old age
Toes
Winter or cold environment
Thumb and little finger
Female (?)