Dense-core matrical mitochondrial bodies in oncocytic adenoma of the thyroid.

Single and/or multiple, round and homogeneous electron-dense bodies occurred within the mitochondrial matrix of a mitochondrion-rich follicular adenoma of the thyroid. These non-membrane-bound bodies varied greatly in size and often occupied a large portion of the mitochondrion. They were present in normal-sized or large mitochondria that exhibited cristolysis and/or dilatation of the intracristal space of the residual cristae and formation of intramatrical myelin figures. The globule-containing mitochondria frequently exhibited bundles of parallel intramatrical filamentous inclusions that were 6 to 8 nm in width and were randomly arranged in clusters. Megamitochondria up to 4.4 micrometers in widest diameter occurred in a frequency of one to two per cell and they displayed similar bundles of filamentous inclusions. This evidence of electron-dense bodies in non-steroid-hormone-secreting cells suggests that the bodies result from nonspecific mitochondrial reaction to injury rather than being inclusions specific for steroid-synthesizing cells.