The nail apparatus consists of a horny “dead” product, the nail plate, and four specialized epithelia: (1) the proximal nail fold, (2) the nail matrix, (3) the nail bed, and (4) the hyponychium (Fig. 89-1A). The nail apparatus develops during the 9th embryonic week from the epidermis of the dorsal tip of the digit as a rectangular area, the nail fold that is delineated by a continuous groove.1 The proximal border of the nail fold extends downward and proximally into the dermis to form the nail matrix primordium. By the 15th week the nail matrix is completely developed and starts to produce the nail plate, which will continue to grow until death. The nail apparatus lies immediately above the periosteum of the distal phalanx. The intimate anatomic relationship between the nail and the bone is responsible for the common occurrence of bone alterations in nail disorders and vice versa. The shape of the distal phalangeal bone also determines the shape and the transverse curvature of the nail.
A. Diagrammatic drawing of a normal nail. B. Keratin gene expression at different locations within the nail. (From De Berker D et al: Keratin expression in the normal nail unit: Markers of regional differentiation. Br J Dermatol 142:89, 2000, with permission.)
Fingernails usually present a longitudinal major axis and toenails a transverse major axis. The ratio between length and width is important for the aesthetic appearance of the nails. The size of the nails varies in the different digits. The biggest nail is that of the first toe, which covers approximately 50% of the dorsum of the digit.
Nails have numerous functions. Fingernails not only contribute to the pleasing appearance of the hands, but are very important in protecting the distal phalanges and enhancing tactile discrimination and the capacity to pick up small objects. They are also widely used for scratching and grooming and are an efficient natural weapon. Toenails protect the distal toes and contribute to pedal biomechanics.
The nail plate is a fully keratinized structure that is continuously produced throughout life (see Fig. 89-1B). It results from maturation and keratinization of the nail matrix epithelium and is firmly attached to the nail bed, which partially contributes to its formation. Proximally and laterally the nail plate is surrounded by the nail folds, which cover its proximal third and lateral margins. At the tip of the digit, the nail plate separates from the underlying tissues at the hyponychium. The nail plate is rectangular, translucent, and transparent. It is curved in both the longitudinal and transverse axes, especially in the toes. The nail plate surface is smooth but frequently shows mild longitudinal ridges that increase with aging (Fig. 89-2). The pattern of these ridges ...