Childhood
In early childhood, the nail plate is thin and may show temporary koilonychia. Because of
the shape of the matrix, some children show ridges that start laterally by the proximal nail
fold and join at a central point just short of the free margin, to give a ‘herringbone’
arrangement of the ridges (chevron nails). In one study 92% of normal infants aged 8–9
weeks showed a single transverse line (Beau’s line) on the finger nails. One child
demonstrated a transverse depression through the whole nail thickness on all 20 digits.

Old age
Many of the changes seen in old age may occur in younger age groups with impaired
arterial blood supply. Elastic tissue changes diffusely affecting the nail bed epidermis are
often seen historically; these changes may
be due to the effects of ultraviolet (UV) radiation, although it has been stated that the nail
plate is an efficient filter of UVB radiation. The whole subungual area in old age may
show thickening of blood vessel walls with vascular elastic tissue fragmentation. Pertinax
bodies are often seen in the nail plate; they are probably remnants of nuclei of
keratinocytes. Nail growth is inversely proportional to age; related to this slower growth,
corneocytes are larger in old age. Since nails tend to thicken with age and some diseases,
it may well be that the volume of nail production per unit of time does not change.
The nail plate becomes paler, dull and opaque with advancing years and white nails
similar to those seen in cirrhosis, uraemia and hypoalbuminaemia may be seen in normal
individuals. Longitudinal ridging is present to some degree in most people after 50 years
of age and this may give a ‘sausage links’ appearance. 

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 (?)

There is a rich arterial blood supply to the nail bed and matrix derived from paired digital
arteries (Figure 1.2). The main supply passes into the pulp space of the distal phalanx
before reaching the dorsum of the digit. The volar digital nerves (Figure 1.2c) are
similarly important in providing nerves to the deep nail apparatus structures. An
accessory blood supply arises further back on the digit and does not enter the pulp space.
There are two main arterial arches (proximal and distal) supplying the nail bed and
matrix, formed from anastomoses of the branches of the digital arteries. In the event of
damage to the main supply in the pulp space, such as might occur with trauma, infection
or scleroderma, there may be sufficient blood from the accessory vessels to permit
normal growth of the nail.
There is a capillary loop system to the whole of the nail fold, but the loops to the roof
and matrix are flatter than those below the exposed nail. There are many arteriovenous
anastomoses below the nail—glomus bodies, which are concerned with heat regulation.
Glomus bodies are important in maintaining acral circulation under cold conditions—
arterioles constrict with cold, but glomus bodies dilate. The nail beds of fingers and toes
contain such bodies (93–501 per cm2). Each glomus is an encapsulated oval organ 300
µm long, made up of a tortuous vessel uniting an artery and venule, a nerve supply and a
capsule; also within the capsules are many cholinergic muscle cells.

Figure 1.2

Digital blood and nerve supply: (a) showing arterial anastomoses; (b)
arterial supply from hand to digits (radio-opaque dye seen in arterises);
(c) major digital arteries and nerve supply.

Nail fold
The proximal nail fold is similar in structure to the adjacent skin but is normally devoid
of dermatoglyphic markings and sebaceous glands. From the distal area of the proximal
nail fold the cuticle reflects on to the surface of the nail plate. The cuticle is composed of
modified stratum corneum and serves to protect the structures at the base of the nail,
particularly the germinative matrix, from environmental insults such as irritants, allergens
and bacterial and fungal pathogens.

Nail matrix
The proximal (dorsal) and distal (intermediate) nail matrix produces the major part of the
nail plate. Like the epidermis of the skin, the matrix possesses a dividing basal layer
producing keratinocytes; these differentiate, harden, die and contribute to the nail plate,
which is thus analogous to the epidermal stratum corneum. The nail matrix keratinocytes
mature and keratinize without keratohyalin (granular layer) formation. Apart from this,
the detailed cytological changes seen in the matrix epithelium under the electron
microscope are essentially the same as in the epidermis.
The nail matrix contains melanocytes in the lowest two cell layers and these donate
pigment to keratinocytes. Under normal circumstances pigment is not visible in the nail
plate of white individuals, but many black people show patchy melanogenesis as linear
longitudinal pigmented bands.

On the great toes, the nail matrix sits like a saddle on the distal
phalanx

Nail bed
The nail bed consists of an epidermal part and an underlying dermal part closely apposed
to the periosteum of the distal phalanx. There is no subcutaneous fat layer in the nail bed,
although scattered dermal fat cells may be visible microscopically. The epidermal layer is
usually no more than two or three cells thick, and the transitional zone from living
keratinocyte to dead ventral nail plate cell is abrupt, occurring in the space of one
horizontal cell layer. As the cells differentiate they are incorporated into the ventral
surface of the nail plate and move distally with this layer.
The nail bed dermal fibrous tissue network is mainly oriented vertically, being directly
attached to phalangeal periosteum and the epidermal basal lamina. Within the connective
tissue network lie blood vessels, lymphatics, a fine network of elastic fibres and scattered
fat cells; at the distal margin, eccrine sweat glands have been seen.

Nail plate
The nail plate is composed of three horizontal layers: a thin dorsal lamina, the thicker
intermediate lamina and a ventral layer from the nail bed. Microscopically it consists of
flattened, dead squamous cells closely apposed to each other. In older people acidophilic
masses are occasionally seen, called ‘pertinax bodies’.
The nail plate is rich in calcium, found as the phosphate in hydroxyapatite crystals; it is
bound to phospholipids intracellularly. The relevance of other elements which are present
in smaller amounts, such as copper, manganese, zinc and iron, is not exactly known.
Calcium exists in a concentration of 0.1% by weight, 10 times greater than in hair.
Calcium does not significantly contribute to the hardness of the nail. Nail hardness is
mainly due to dense sulphur protein from the matrix, which contrasts with the relatively
soft keratin of the epidermis. The normal curvature of the nail relates to the shape of the
underlying phalangeal bone to which the nail plate is directly bonded via the vertical
connective tissue attachment between the subungual epithelium and the periosteum.

The component parts of the nail apparatus are shown in Figure 1.1. The
rectangular nail plate is the largest structure, resting on and firmly attached to
the nail bed and the underlying bones; it is less firmly attached proximally, apart
from the posterolateral corners. Approximately one-quarter of the nail is covered
by the proximal nail fold, while a narrow margin of the sides of the nail plate is
often occluded by the lateral nail folds. Underlying the proximal part of the nail
is the white lunula (‘half-moon’ or lunule); this area represents the most distal
region of the matrix. The natural shape of the free margin of the nail is the same
as the contour of the distal border of the lunula. The nail plate distal to the
lunula is usually pink owing to its translucency, which allows the redness of the
vascular nail bed to be seen through it. The proximal nail fold has two epithelial
surfaces, dorsal and ventral; at the junction of the two the cuticle projects
distally on to the nail surface. The lateral nail folds are in continuity with the
skin on the sides of the digit laterally, and medially they are joined by the nail
bed.
The nail matrix can be subdivided into proximal (or dorsal) and distal (or
intermediate) sections, the latter underlying the nail plate to the distal border of
the lunula. It is now generally considered that the nail bed contributes to the
deep surface of the nail plate (ventral matrix), although this thin, soft, deep
component plays little part in the functional integrity of the nail plate in its distal
part. At the point of separation of the nail plate from the nail bed, the proximal
part of the hyponychium may be modified as the solehorn. In hooved animals
this is the site of hard keratin hoof formation—it may also be the source of hard,
distal subungual hyperkeratosis in diseases such as psoriasis and pachyonychia
congenita. Beyond the solehorn region the hyponychium terminates at the distal
nail groove; the tip of the digit beyond this ridge assumes the structure of the
epidermis elsewhere.
When the attached nail plate is viewed from above, several distinct areas may
be visible, such as the proximal lunula and the larger pink zone. On close
examination two further distal zones can often be identified: the distal
yellowish-white margin, and immediately proximal to this the onychodermal
band. The latter is a barely perceptible, narrow transverse band 0.5–1.5 mm
wide. The exact anatomical basis for the onychodermal (onychocorneal) band is
not known but it appears to have a separate blood supply from that of the main
body of the nail bed; if the tip of the finger is pressed firmly, the band and an
area just proximal to it blanch, and if the pressure is repeated several times the
band reddens.

Figure 1.1

(a), (b) Nail apparatus structures; (c) longitudinal nail biopsy
section, oriented to equate with (b).