[Quantitative light and electron microscopy studies of myofibrils and mitochondria of dog and chicken hearts]

Ventricular myocytes are characterized by a central nucleus which is surrounded by longitudinal myofibrils and a mostly column-like longitudinal distribution of mitochondria. This study was designed to investigate the subcellular distribution of organelles in close proximity to the nucleus. Of particular interest was the question of whether volume density of organelles close to the nucleus is maintained and, as a consequence, if the cellular diameter at the site of the nucleus is increased according to the dimension of the nucleus. Therefore, dog and chicken hearts were subjected to perfusion fixation and sections of myocytes were studied in longitudinal and cross-sectional axes by light and electron microscopy. Volume density of organelles was analyzed on longitudinal sections according to Rosiwal's principle and on cross-sections by the point-counting method and a digitalized imaging system. We found that the cross-sectional diameter of cardiac myocytes is slightly wider at the site of the nucleus according to the dimension of the nucleus. The volume density of myofibrils and mitochondria is similar in most subcellular fractions except at the nuclear poles where mitochondria were significantly more abundant. Thus, no significant disturbance of the organelle distribution is observed at the site of the nucleus.