Ultrastructural observations on mitochondria and microvesicles in renal oncocytoma, chromophobe renal cell carcinoma, and eosinophilic variant of conventional (clear cell) renal cell carcinoma

On light microscopic examination, the morphologically overlapping features of granular eosinophilic cytoplasm in renal oncocytoma and the eosinophilic variants of chromophobe renal cell carcinoma and conventional (clear cell) renal cell carcinoma may pose difficulties in diagnosis. We investigated t he ultrastructure of 5 renal oncocytomas, 7 eosinophilic variants of chromo phobe renal cell carcinoma, and 5 eosinophilic variants of conventional (cl ear cell) renal cell carcinoma. Special attention was paid to mitochondria and microvesicles and interrelations thereof. The electron microscopic Feat ures were correlated with the light microscopic findings. All of the rumors had abundant mitochondria. Although abundant microvesicles were present in all of the chromophobe renal cell carcinomas, scant numbers of microvesicl es were also sometimes present in renal oncocytomas (2 of 5) and in the eos inophilic variant of conventional (clear cell) renal cell carcinoma (1 of 5 ). The mitochondria in all three types of renal neoplasms studied differed in morphology, being predominantly uniform and round with predominantly lam ellar cristae in renal oncocytoma, variable in shape and size with predomin antly tubulocystic cristae in chromophobe renal cell carcinoma, and swollen and pleomorphic with rarefied matrix and attenuated cristae in the eosinop hilic variant of conventional (clear cell) renal cell carcinoma. Variable n umbers of mitochondria in all of the chromophobe renal cell carcinomas had outpouchings of the outer membranes, some of which carried parts of inner m embrane within them. These outpouchings closely resembled the nearby cytopl asmic microvesicles, as did the tubulocystic cristae of the mitochondria. S ome microvesicles contained homogeneous, electron-dense, finely granular ma trix, similar to that seen in mitochondria. In one of seven chromophobe ren al cell carcinomas, microvesicles were present in rough endoplasmic reticul um, and in two others, mitochondria were present within some vesicles. Thes e features strongly suggest a close relationship between the microvesicles and mitochondria. Based on the role of vesicle formation in normal mitochon driogenesis, and some of our observations, we propose that defective mitoch ondriogenesis may be the source of microvesicles in chromophobe renal cell carcinomas.