Quantitive Analysis of Ultrastructural Changes in Developing Rat Cardiac Muscle During Normal Growth and During Acute Volume Load

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  • QUANTITATIVE ANALYSIS OF ULTRASTRUCTURAL CHANGES IN DEVELOPING RAT CARDIAC MUSCLE DURING NORMAL GROWTH AND DURING ACUTE VOLUME LOAD
  • Quantitive Analysis of Ultrastructural

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1. In comparison with adult rats, the myocardial cells of young post-natal rats contain more ribosomes, glycogen particles and roughsurfaced endoplasmic reticulums, but fewer mitochondria and myofibrils. The mean volume percentages of mitochondria both atria and both ventricles, those of myofibrils are 44 to 48%, and those of glycogen particles are 14 to 18%. 2. In comparison with young post-natal rats, the myocardial cells of 7-day-old rats contain better-developed mitochondria and myofibrils, but fewer ribosomes, glycogen particles and rough-surfaced endoplasmic reticulums. 3. The volume percentages of mitochondria, myofibrils or glycogen particles in 3-month-old rats are almost the same as those of adult rats. Namely, the mean total areas occupied in the sarcoplasm by mitochondria in the four chambers are 32 to 36%, those by myofibrils are 53 to 58%, and those by glycogen particles are 4 to 8%. 4. Thus, for the three main intracellular components; mitochondria myofibrils, and glycogen particles; there are no significant differnces among the mean contents in the four chambers at any age. 5. In spite of abrupt circulatory changes at birth, both damages of myofibrils and intracellular organs and intracellular edema are seldom seen in the myocardium of young post-natal rats. Such shows that the young post-natal rat cardiac muscle has sufficient tolerance to these natural loads. 6. Mitosis was found in the cardiac muscle cells of an 8-day-old rat and cell proliferation after birth was confirmed. 7. Throughout the developmental periods, the minimum diameters of the nuclei in both atria and both ventricles did not greatly alter to become significantly different. 8. The cell diameter of the left atrium of 0-day-old rats was significantly smaller than that of the right atrium, right ventricle or left ventricle. The cell diameters of the right atrium and right ventricle were not significantly altered until the 7th day after birth, but that of the left atrium increased markedly for the same period, and then, became equal to that of the right atrium or right ventricle. Further, the cell diameter of the left ventricle increased markedly. After that period, the cell diameters of both atria and both ventricles increased prominently with increasing age. The increase was more prominent in the ventricular than in the atrial myocardiums. 9. Mitochondrial destruction and intracellular edema were found in the myocardium of young post-natal rats after volume load by blood equalling the whole blood volume. In the myocardium of adult rats during the same volume load, however, the above damages were only slight, and numerous enlarged mitochondria with increased number of cristae occupied a larger intracellular space. These results show that the tolerance of young post-natal cardiac muscle is poorer than that of adult cardiac muscle.

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