The pervasive misunderstanding of the nail unit as a skin equivalent rather than as an adnexal unit such as the hair follicle has complicated the evaluation and diagnosis of nail diseases.1 After recognizing that the matrical and bed epithelia and stroma are not epidermis and dermis, a better understanding of the microscopic findings in nail diseases has emerged.
The nail can be considered to resemble a flattened, cubist-like hair follicle that is asymmetric (Fig. 36-1). The plate is laminar and enclosed by lip-shaped sides of unequal length. The plate-forming matrix is lateralized rather than enclosing the nail root (Fig. 36-2A). The intermediary epithelia surrounding the matrix, especially the bed epithelium, are its products as well, as are ultimately the resulting cuticular keratins that ensheath the plate (Fig. 36-2B).2
Clinical anatomy of the nail and diagram of nail structures. (A) The morphology of the plate is the result of intimate accommodation of the nail structures to the subjacent bone. The nail root rests over the proximal epiphysis of the distal phalanx, and the distal groove (at the site of onychodermal band and hyponychium) arches around (and actually inserts, by specialized nail ligaments, on) the tuberositas unguium. (B) Diagram of nail structures.
Histology of the nail (in a newborn). (A) The matrix epithelium is stratified and produces an eosinophobic nail plate, mirrored by concurrent keratin of the ventral portion of the proximal nail fold (true cuticle). (B) The nail bed epithelium is less stratified than the matrical one, with relatively monotonous upper and lower keratinocytes. (C) The onychodermal nail band occurs where the bed epithelium clashes with the epidermis of the hyponychium. The entirety of the bed epithelium keratinizes and is extruded as the solehorn (ventral cuticle).
Regarding the dynamics of migration of the nail components, every layer of the nail unit moves forward alongside the plate in unison. Thus, the plate does not glide over the bed epithelium but is firmly attached to it in a parallel course. The bed ends at a predetermined site, the onychodermal band, which may be recognized clinically (Fig. 36-2C) and keratinizes fully in a zone known histologically as the solehorn. This keratinization may be described as onycholemmal keratinization.3 On the opposite side, the true cuticle, which originates on the ventral side of the proximal nail fold, apposes the plate as it emerges, thus sealing the virtual space that otherwise would occur.