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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.
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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
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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.
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All of the described onychal structures are encircled by cutaneous structures more ...