**2. Histopathologic nail reactions**

In contrast to skin, the nail demonstrates a variety of specific reactions that are sometimes opposite to the rest of the skin. Whereas irritation and a number of inflammatory reactions cause parakeratosis in skin they may induce a granular layer in the nail, some psoriatic reactions included. Several dermatoses that are not characterized by spongiosis, may exhibit marked intercellular oedema of matrix and nail bed epithelium, e.g. in ungual lichen planus and psoriasis.

As the dorsal layer of the nail plate is produced by the most proximal portion of the matrix any alterations of the proximal matrix will translate into changes of the nail surface and uppermost layers. The bulk of the nail is produced by the middle matrix portion; alterations here will modify the nail plate both in its thickness as well as optical coherence. When the distal matrix is involved the resulting nail alterations will appear at the undersurface of the nail (Figure 2). All matrix-derived alterations will grow out with the nail. In contrast, nail bed alterations will be seen through the nail, but will not be integrated into the nail. As the nail bed keratin moves slower than the nail plate nail bed alterations remain longer or may even appear non-migratory.

Psoriasis of the nails induces characteristic histopathological changes that are pathognomonic in the vast majority of cases; however, in the beginning and with less pronounced changes it may mimic a spongiotic dermatitis or may be indistinguishable from onychomycosis if there is no proof of fungal invasion.
