Unna's and Miescher's nevi: two different types of intradermal nevus: hypothesis concerning their histogenesis.

In 1991, we tentatively introduced the classification of Ackerman and Magana-García for acquired melanocytic nevi in our laboratory. We soon realized that every acquired intradermal melanocytic nevus might be easily classified into either Unna's or Miescher's patterns and that this classification had both clinical implications and significant histological differences. The decisive discriminative feature between Unna's and Miescher's nevi is that Unna's nevus is an almost purely adventitial lesion confined to expanded papillary dermis and, many times, to the perifollicular dermis too. In Miescher's nevus melanocytes diffusely infiltrate both adventitial and reticular dermis in a wedge-shaped pattern. With these concepts in mind, every acquired intradermal melanocytic nevus can be easily classified as either Unna's or Miescher's. We studied 751 acquired melanocytic nevi; 458 (61%) of them were intradermal; of these, 234 were Unna's nevi and 224 were Miescher's nevi. Eighty- three per cent of the nevi from the head and neck were intradermal nevi, whereas on the trunk and limbs junction and compound nevi were the most frequent (56%). When intradermal nevi were divided into Unna's and Miescher's patterns, it resulted that 91% of Miescher's nevi located on the face and 94% of intradermal nevi on the face were Miescher's nevi. In contradistinction, 87% of the Unna's nevi located on the neck, trunk, and limbs, and 96% of intradermal nevi from these locations were Unna's nevi. Only on the scalp was there no clear predominance of one type of intradermal nevus. A series of other histological characteristics were significantly predominant (P = 0.000) in either Unna's or Miescher's nevi. Unna's nevi had more: junctional nests above the intradermal component (40% versus 20%), a radial pattern of intradermal nests (38% versus 0%), vascular-like clefts lined by nevus cells (48% versus 4%), and in depth maturation (94% versus 0%). Miescher's nevi had more: pilosebaceous follicles within the nevus (100% versus 51%), subnevis folliculitis (12% versus 1%), large isolated melanocytes along the basal epidermal layer (47% versus 11%), multinucleated nevocytes (89% versus 44%), and adipocytes within the nevus (53% versus 11%). In conclusion, Unna's and Miescher's nevi are 2 subsets of acquired melanocytic nevus with clinical implications and significant histological differences. A histogenetic hypothesis is proposed on the basis of their histological structure.