Electron Microscopic Observations of Hypertrophied Myocardium of Rat Produced by Injection of Monocrotaline

The right ventricular myocardium of the cor pulmonare in the rat induced by subcutaneous single injection of monocrotallne has been studied by electron microscope. The hypertrophy and wearing of myocardium was sequentially categorized into (1) preparatory stage, (2) hypertrophied stage, and (3) damaged stage. Slightly enlarged and injured mitochondria and incomplete division of mitochondria were noticed in the first stage. An increase in number of mitochondria, appearance of giant mitochondria, many free ribosomes and small bundles of myofilaments were recognized in the second stage. In the third stage, mitochondria located in the subsarcolemmal region and between the myofibrils tended to migrate to the central portion of the muscle cell, and the myofibrils were simultaneously pushed away towards the plasma membrane. A large number of spherical, electron‐opaque granules, not containing acid phosphatase, were found in the enlarged central sarcoplasmic core. The transverse cell boundary was expansively widened at the intermediate junction and its space contained many small membrane‐limited particles. Injured cells, reduced in size, were isolated and localized in the tissue space. After degeneration of this cell, it is finally replaced by collagen fibers.