Peculiar cytoplasmic inclusions in oncocytic adrenal cortical tumors: an electron microscopic observation.

Two cases of an oncocytic adrenal cortical tumor that contained peculiar cytoplasmic crystalline inclusions in the tumor cells are presented. The patients were 49- and 72-year-old females without clinical and biochemical evidence of adrenal cortical or medullary dysfunction. The adrenal tumors weighed 80 and 200 g each. These crystalline inclusions were present in groups of longitudinal profiles or clusters of crossly cut aggregates. They appeared in clusters of membrane-bound columns. On longitudinal sections, they appeared as rigid rods of homogenous density measuring 36 nm in width, but when they were cut transversely their paracrystalline nature became apparent. They were composed of closely packed microtubules in rectangular blocks. The microtubules measured 12.5 nm with a hollow center measuring 4.2 nm. The inclusions were within the membrane-bound cisterna of rough-surfaced endoplasmic reticulum. The significance of these inclusions is not clearly understood; however, they have been seen only in adrenal cortical tumors and their presence may be helpful in the differential diagnosis of adrenal oncocytic tumors. One patient presented with a tumor in which gross and microscopic appearance was compatible with a pheochromocytoma. This case exhibited an oncocytic appearance and pronounced cellular pleomorphism. Ultrastructural studies were necessary to recognize the tumor cells as cortical cells. The tumor cells contained abundant mitochondria with tubular cristae, paranuclear parallel stacks of granular endoplasmic reticulum, and relatively prominent smooth endoplasmic reticulum. These features are typical of adrenocortical cells. In addition, frequent tumor cells contained the peculiar cytoplasmic inclusions herein described.