Classification of melanocytic nevi: do nodular and flat nevi develop differently?

Previous classifications of melanocytic nevi have not been generally accepted for various reasons. The classification by Ackerman and Magana-Garcia (Am J Dermatopathol 1990;12:193) has also been subject to criticism, even though it is the first taking into account of both clinical and histological aspects of these tumors. To reproduce the findings by Ackerman and Magana-Garcia, the following findings were observed: (a) cases with features of two different types were occasionally seen--33 of 185 cases (17.3%) of nevi of Miescher's and Unna's type showed features of both types and 6 of 205 (3%) flat nevi had a slight to pronounced papillomatous surface--(b) contrary to Unna's theory of "Abtropfung," no junctional nevi of Unna's and Miescher's type and no dermal nevi of cases with a flat configuration were seen. It is concluded that the main differences among melanocytic nevi were seen between flat and nodular nevi, showing (a) different clinical configurations (b) different histological features, (c) possibly a different histogenesis, and (d) different malignant potentials.