Which direction do nevus cells move? Abtropfung reexamined.

In 1893, Unna published his theory of Abtropfung in which he suggested that melanocytic nevus cells originate in the epidermis and drop off into the dermis. We studied 3,534 nevi from patients of all ages to reassess this almost sacred concept. If Unna was correct, one would expect that in childhood most nevi would be junctional, while in late adult life almost all nevi would be intradermal. In our series, no child under age 10 had a purely junctional nevus, 52% had compound nevi, and 48% had dermal nevi. In patients older than age 60, 12% had junctional nevi, 23% had compound nevi, and 65% had dermal nevi. Our data fail to support the concept of Abtropfung; they fit better with the contradictory theory of upward migration of nevus cells.