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.
Vimentin was mostly negative, however, some cells showed faintly positive cytoplasm. HMB-45 was also positive in many of the epithelial cells. 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 .