Histogenesis of congenital and acquired melanocytic nevi based on histological study of lesion size and thickness.

The histogenesis of melanocytic nevi is poorly understood. It is important to determine the differences and similarities in histogenesis between congenital and acquired nevi. To clarify the histogenic differences between acquired melanocytic nevi (AMN) and congenital melanocytic nevi (CMN), diameter and depth of nevus cells (tumor thickness) were examined in histological specimens from 80 cases of CMN and 71 cases of AMN, and these nevi were classified according to Mark's pathological CMN criteria. In all cases, giant CMN nevus cells were found in the lower marginal portion of excised specimens. The mean diameter and lesional thickness were significantly higher in CMN than in AMN. AMN diameter showed a significant correlation (r = 0.567, P < 0.05) with lesional thickness, while no such relation was observed in CMN. In addition, a significant correlation between lesion diameter and thickness was observed in small (<10 mm) non-Mark's type CMN (r = 0.626, P < 0.05). CMN may be classified into three subtypes: (i) caused by increased proliferation of melanoblasts during the course of migration from the neural crest to the epidermis; (ii) proliferation of nevus cells after arrival at the epidermis, and nevus cell distribution affected by adnexa and dermal differentiation; and (iii) arising after completion of skin development before birth.