H&E stained sections show enlongated papillary fronds (example 1, example 2, example 3) covered by a layer of low-cuboidal to columnar epithelial cells (example 1, example 2), some with cilia. The fronds contain abundant stroma and in many areas the epithelium plunges into the core of the papillae forming complex gland-like structures. Most of the epithelial cells on the surface, as well as those within the gland-like structures are densely pigmented. This pigment is fine to coarsely granular and is also present in some stromal cells. Cytologically all cells are benign and there are no mitoses. In a few areas the epithelium is several cells thick.
Trichrome stains demonstrate that the stroma of the cores is composed primarily of fibrous connective tissue, presumably collagen. Iron stains were negative and Fontana-Masson stains positive, confirming that the pigment was melanin. PAS stains demonstrate a well-delineated basement membrane under most epithelial cells.
Immunohistochemical studies (using AEC) demonstrate that nearly all epithelial cells are S-100 positive and that there are focal epithelial cells postive for GFAP (example 1, example 2). Vimentin was mostly negative, however, some cells showed faintly positive cytoplasm. An AE1/AE3 cytokeratin cocktail was negative, however, other cytokeratin immunostains such as CAM 5.2 and F411 (which recognize some cytokeratins not covered by AE1/AE3) were strongly positive. HMB-45 was also positive in many of the epithelial cells.
Electron microscopy revealed pigmented epithelial cells containing mature and compound melanosomes (example 1, example 2). Other structures were also present which could represent atypical pre-melanosomes (example 1, example 2). Some cells were heavily laden with melanosomes. All epithelial cells had a basal lamina and most displayed well developed desmosomes. Many cells also displayed microvilli and cilia.