Fig. 4

Deep penetrating (beta-catenin activated) nevus/melanocytoma. a Low power view shows extension of the melanocytoma into the reticular dermis with melanocytes extending around adnexal structures. b Characteristic cytology is apparent at higher magnification. The melanocytes have moderate amounts of vacuolated and lightly pigmented cytoplasm and slight nuclear pleomorphism. They are arrayed in oval to fusiform nests with a somewhat plexiform pattern. There are scattered melanophages. c Beta-catenin immunohistochemistry demonstrates strong cytoplasmic (and nuclear) staining of the melanocytes. The staining is uniform throughout the melanocytoma. d A superficial focus of beta-catenin-activated melanocytoma arising within a congenital pattern common acquired nevus, low power view. e Beta-catenin immunohistochemistry shows intense cytoplasmic (and nuclear) staining of fusiform nests of melanocytes. In contrast, within the common acquired nevus the superficial melanocytes show patchy positivity and melanocytes deeper in the reticular dermis show little positivity. f High power view demonstrates two intermixed populations of melanocytes with those of the beta-catenin-activated melanocytoma showing features similar to those in b and adjacent small round melanocytes with scant, pale cytoplasm. g The two populations of melanocytes show distinct patterns of beta-catenin expression