DISTRIBUTION OF MYOFIBRILS AND MITOCHONDR IA IN VENTRICULAR MYOCYTES - A QUANTITATIVE LIGHT AND ELECTRON-MICROSCOPIC STUDY IN DOG AND CHICKEN HEARTS

Ventricular myocytes are characterized by a central nucleus which is s urrounded by longitudinal myofibrils and a mostly column-like longitud inal distribution of mitochondria. This study was designed to investig ate the subcellular distribution of organelles in close proximity to t he nucleus. Of particular interest was the question of whether volume density of organelles close to the nucleus is maintained and, as a con sequence, if the cellular diameter at the site of the nucleus is incre ased according to the dimension of the nucleus. Therefore, dog and chi cken hearts were subjected to perfusion fixation and sections of myocy tes were studied in longitudinal and cross-sectional axes by light and electron microscopy. Volume density of organelles was analyzed on lon gitudinal sections according to Rosiwal's principle and on cross-secti ons by the point-counting method and a digitalized imaging system. We found that the cross-sectional diameter of cardiac myocytes is slightl y wider at the site of the nucleus according to the dimension of the n ucleus. The volume density of myofibrils and mitochondria is similar i n most subcellular fractions except at the nuclear poles where mitocho ndria were significantly more abundant. Thus, no significant disturban ce of the organelle distribution is observed at the site of the nucleu s.