Ultrastructural study of a black insulinoma.

A case of black insulinoma is reported. The color was due to a cytoplasmic pigment. Immunostaining for neuron-specific enolase and chromogranin was positive in the tumor cells, and the pigment granules themselves reacted with the chromogranin antibody. Numerous beta cell type dense core granules as well as atypical granules were found by electron microscopy. An important finding was that the dense core granules contribute to the lipofuscin pigment formation.