Analytical electron microscopic study of mitochondrial inclusions in canine myocardial infarcts

An analytical electron microscopic study, utilizing scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy, was made of two types of mitochondrial inclusions identified in canine myocardial infarcts. The data were obtained from thin sections of tissues that were fixed in aldehyde, osmicated and embedded in epoxy resin. Calcium peaks of variable intensity were detected in inclusions which contained very electron-dense spicular material and which were localized to muscle cells at the peripheries of the infarcts. These findings indicate that the spicular inclusions represent early stages in the process of mitochondrial calcification in myocardial infarcts. In contrast, calcium or other trace elements were not detected in moderately electron-dense amorphous inclusions which were present in mitochondria of muscle cells throughout the infarcts. With the tissue preparative techniques employed, the possibility cannot be excluded that the amorphous inclusions contained calcium, either in small amounts or in a readily diffusable state, in vivo. The data, however, are in accord with the previously advanced hypothesis that the amorphous inclusions represent precipitates of denatured mitochondrial protein formed during the evolution of irreversible cellular injury. This study provides further evidence that analytical electron microscopy can yield important information regarding the nature of various inclusions occurring in normal and diseased tissues.