Characteristic distribution of melanin columns in the cornified layer of acquired acral nevus: an important clue for histopathologic differentiation from early acral melanoma.

Clinical and histopathologic differentiation between early acral melanoma and acral nevus is often difficult. Dermoscopy is helpful in this differentiation. On dermoscopy, early acral melanoma shows the parallel ridge pattern showing band-like pigmentation on the ridges of the surface skin markings, whereas a representative dermoscopic pattern in acquired acral nevus is the parallel furrow pattern showing parallel linear pigmentation along the surface furrows. The parallel furrow pattern suggests that melanocytes of acral nevus preferentially proliferate in the crista profunda limitans, an epidermal rete ridge underlying the surface furrow. In the present study, however, we found that in 13 of 18 acquired acral nevi, proliferation of melanocytes were detected not only in the crista profunda limitans but also in the crista profunda intermedia (CPI), an epidermal rete ridge underlying the surface ridge. Very interestingly, Fontana-Masson staining of these acral nevi revealed that even when proliferation of melanocytes was prominent in the CPI, melanin granules in the cornified layer were observed as regular melanin columns situated under the surface furrows and were hardly detected under the surface ridges. These findings indicate that in acral nevus, melanin granules produced by melanocytes in the CPI are not transferred to the upper epidermis. Hence, we must be careful not to overdiagnose an acral melanocytic lesion in which an increased number of melanocytes are detected in the CPI. Even in such a case, if melanin granules in the cornified layer are detected as melanin columns regularly distributed under the surface furrows, the lesion is strongly suggested to be a benign acral nevus.