[Mitochondrial apoptosis].

Silver staining methods, when selective, yield a high-contrast and high-resolution image in optical microscopy. A classical method for silver impregnation of mitochondria has been applied to liver tissue and reveals a marked heterogeneity among mitochondria in single cells. This heterogeneity can be detected in the optical microscope but is even more evident at the ultrastructural level. The differences in staining intensity may reflect different stages in the mitochondrial life cycle. The most lightly stained mitochondria and the unstained mitochondria also observed with Electron microscopy may represent the earliest phase of the mitochondrial life cycle. On the other hand, the progressively darker staining and final appearance of shrunken mitochondria may reflect the autodissolution process of these organelles. Such a process of mitochondrial apoptosis might make sense as a preventive pathway for elimination of damaged mitochondria before cytC or other proapoptotic factors could be liberated. This method reveals a marker of mitochondrial age.