The nail is mainly composed of the hard nail plate arising from a matrix. The shape of the nail is very variable in different persons; it is roughly rectangular and flat in shape; The nail bed has a pink color that can be seen because of the transparency of the plate and its extensive vascular network.
A whitish crescent -shaped lunula projecting from under the proximal nail folds is observed in the thumbs, uncommonly in the other fingers and in the large toenails. The lunula is the more distal portion of the matrix and determines the shape of the free edge of the nail plate. Its color is due in part to the effect of light scattered by the nucleated cells of the matrix and in part to the thick layer of the epithelial cells making up the matrix.
As the nail plate emerges from the matrix, its lateral and proximal borders are enveloped by folds of skin termed lateral and proximal nail folds. The skin underlying the free end of the nail is referred to as the hyponichium and is contiguous with the skin on the tip of the finger.
In humans, nails grow at an average rate of 3 mm a month. The growths of fingernails and toenails have different speeds. Complete regrowth of fingernails require 3 to 6 months while complete toenail regrowth needs 12 to 18 months. The growth rate is related to the length of the terminal phalanges; the nail of the little finger grows slower than the nail of the index finger. They grow faster in summer than in the other season. Contrary to popular belief, they do not continue to grow after death; it is the severe postmortem drying and shrinking of the soft tissue around the nail plate that give the illusion of a nail growth.
The main functions of the nails are to protect the end of the digit and to help in grasping objects.
In embryos, at 10 weeks of age, the surface of the fingertips is composed of undifferentiated epithelial cells that constitutes the nail field; at this stage, the matrix primordium consists of a wedge of basal-like cells that is growing proximally and diagonally into the deeper tissue of the phalanx at a point near the interphalangeal joint and that will continue to develop till newborn age or early infancy. In embryos, from 13 weeks up to 32 weeks, the lunula starts distally then proximally to differentiate from the more distal and older portions of the matrix primordium and produces the nail plate. The process of differentiation of the basal cells of the matrix consists of a flattening of the cells, a fragmentation of the nuclei, a condensation of the cytoplasm to form horny cells strongly adherent to one another. In contrast with what is observed in the hair, the cell membranes are prominent in the final horny layer. The region of fragmentation of nuclei and cytoplasm condensation is named keratogenous zone.
The area extends from the lunula to the hyponychium. In the adults, it does not contribute to the nail plate, with the exception of few horny cells adherent to the ventral face of the nail plate. The nail bed epidermis differentiates vertically and there are no keratohyalin granules in the adults while they are present in the embryos until 17 to 20 weeks of development; they disappear with covering of the nail bed by the growing nail plate.
The hyponichium is the portion of the epidermis under the free edge of the nail which extends from the nail bed to the distal groove. It constitutes a seal that protects the nail bed. Its differentiation process is like in the epidermis elsewhere with a granular layer containing abundant keratohyaline granules. The onychodermal band is the seal found under the free edge, between the nail plate and the hyponychium, where the nail bed ends.
The proximal nail fold is composed of two layers of epidermis; the dorsal layer constitutes the dorsum of the finger epidermis and the ventral layer overlies the newly formed nail plate. They differentiate as the epidermis elsewhere; the horny layer of the ventral portion is adherent to the surface of the newly formed nail plate and moves distally. The eponychium is a small band of living epithelium that extend from the proximal nail wall onto the base of the nail. It is the end of the proximal fold that sheds a non living almost invisible epidermal layer, named the cuticle which forms with the eponychium a protective seal at the proximal part of the nail plate.
The nail plate is composed primarily of proteins, mainly fibrous keratins consisting of both epidermal and hair keratins, the latter comprising more than 90% of the total. The keratin filaments lie parallel to the surface of the nail and are oriented in a direction parallel to the growth of the nail. The lipid content of the nail plate is less than 5%.
The nail has a very porous structure. The diffusion constant for water in the nail is 100 times that of epidermis and the nail is rapidly hydrated with water.