Histopathologic findings in Unna's nevus suggest it is a tardive congenital nevus.

According to A. Bernard Ackerman's clinical histopathologic classification of nevi, the essential histopathologic finding of an Unna's nevus is localization of melanocytic nevus cells to a markedly thickened papillary dermis of an exophytic lesion. In this study, we examined 94 completely resected Unna's nevi in which several sections of the same lesion could be examined. We examined that melanocytic nevus cells were not just localized to the exophytic portion but were also commonly distributed below it (81 lesions, 86.2%). Melanocytic nevus cells were found in a periadnexal distribution in most of the lesions in which they extended below the exophytic part and were most frequently noted around hair follicles (60 lesions, 63.8%), then around eccrine ducts (43 lesions, 45.7%), and least frequently around sebaceous glands or ducts (36 lesions, 38.3%). Nests of melanocytic nevus cells were present in follicular epithelium in 7 lesions, sebaceous ducts in 1 lesion, and eccrine ducts in 1 lesion. Moreover, type A nevus cells containing melanin granules were observed in 16 lesions (17.0%) when they were distributed around hair follicles, in 8 lesions (8.5%) when distributed around sebaceous glands or ducts, and in 1 lesion (1.1%) when distributed around eccrine ducts. Although Unna's nevus is clinically an acquired nevus, it has many histopathologic characteristics of a congenital melanocytic nevus and is therefore likely a tardive congenital lesion.