Nails

Embryology and Normal Structure

The nails begin to develop at the distal end of the digits at about 10 weeks of gestation. They appear as thickened areas of the epidermis (Fig. 11.7). These nail fields are surrounded by a fold of the epidermis, the nail fold. Cells from the proximal part (the nail bed) of the nail fold grow over the nail field and become keratinized, forming the nail or nail plate. New nail growth is produced by the nail bed in the proximal part of the nail field. The nail bed can be seen in some individuals as a white crescent at the base of the nail. The developing nail is covered by an epidermal layer, the eponychium, which later degenerates, except at the base of the nail, where it is called the cuticle. The fingernail plate grows to reach the fingertip at about 32 weeks of gestation. The toenail plate reaches the tip of the toe at 36 weeks. The size of the nail field is influenced by the size of the underlying distal phalanx.

In the newborn, the mean width of the index fingernail is 5.0 ± 0 and the length is 3.5 ± 0.3 mm.

With aging, the convexity of the nail plate increases, and the nail plate thickens and may develop ridges. These changes can sometimes be confused with a nail infection. The rate of nail growth is more rapid in warm climate, while nail growth slows down with infections and age.

Lateral nail fold 10 weeks

Nail field

Nail plate 14 weeks

Lateral nail fold 10 weeks

Nail plate 14 weeks

Nail field

Figure 11.7 Development of nails. From Moore (1982), by permission.

Nail Size, Shape, Quality, and Color

The size of the nail in relation to the nail bed, the shape (e.g., round, oval, long, clubbed, broad [Fig. 11.8]), the quality (e.g., thick, thin, brittle, splitting, ridged, pitted), and any discoloration should be recorded. As with other body structures, photographs are often useful.

Specific changes can be seen in certain syndromes:

Longitudinal ridging occurs in cranio-fronto-nasal syndrome.

Longitudinal ridging and nail dystrophy with the thumbnails most severely affected, including absence of the lateral half of the thumbnail, may be seen in nail-patella syndrome.

Thickened, friable, and darkened fingernails and toenails are seen in pachyonychia congenita.

Ungual fibromata are seen in tuberous sclerosis.

Figure 11.8 Normal variation of fingernail configuration. From Martin and Saller (1962), by permission.

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