Carciac muscle cytoplasmic and nuclear development during canine neonatal growth

Morphological aspects of cardiac myocytes during neonatal growth were studied in isolated cell preparations from 23 dog hearts (newborn to 38 weeks old). Additional hearts were perfused with glutaraldehyde for electron microscopic study. At least 200 isolated cells in stained smears of each ventricle were examined for nuclear number, and at least 25 cells were measured with an eyepiece micrometer to calculate cell volume. At birth, 96-100% of myocytes contained a single nucleus. By 2 weeks, 15% of cells contained two to four nuclei; multiple nuclei were present in 55% of myocytes at 4 weeks, in 85% at 6-10 weeks, and in 55-60% at 15, 28, and 38 weeks. Although left ventricular weight increased lenearly from birth, there was no increase in single nucleated cell size until 4-6 weeks, indicating cellular hyperplasia; by 38 weeks single nucleated cells were 9 times the volume at birth. Double nucleated cells steadily increased in size, reaching a volume at 38 weeks 19 times that of newborn myocytes. At birth, myocytes were small and spindle-shaped; intercalated discs were poorly developed, but lateral connections between cells were common. As the cells increased in size, the lateral attachment areas appeared to move to the cell ends to form the intercalated discs. Myofibrils appeared to be assembled at the lateral connections and at the intercalated discs. Hyperplasia is the prevalent means of cell growth at birth in the dog and is gradually replaced by cell hypertrophy from 2 to 6 weeks of age. Double nucleated cells occur in hypertrophying cells by karyokinesis without subsequent cytokinesis, perhaps in response to the intense protein synthetic demands of normal growth.