Hypertrophied muscle cells of the right ventricles obtained from 12 patients with congential malformation of the heart (VSD, TF, A‐V canal and AASV) have been studied by electron microscope.The process of hypertrophy of cardiac muscle cells was almost identical among these cardiac diseases.Slightly hypertrophied muscle cells are characterized by the significant increase of electron lucent cytoplasmic area containing numerous free ribosomes. As the muscle cells become more hypertrophied, synthesis of myofilaments is frequently observed in the subsarcolemmal and Interfibrillar cytoplasmic regions rich in free ribosomes. Numerous dense granules, 150 to 400 mμin diameter, are noticed with Intimate relation to the well developed Golgi complex and a few elements of rough‐surfaced endoplasmic reticulum. In the markedly hypertrophied muscle cells, the total volume of the contractile elements and the number of mitochondria are conspicuously increased. The central sarcoplasmic core of these cells is expansively extended and contains numerous mitochondria. The synthesis of myofilaments is not recognized in this stage. The appearance of lipofuscin pigments and the enlargement of the nucleus are already noticed in the early period of muscle hypertr
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