Persistence of neoangiogenesis and cardiomyocyte divisions in right ventricular myocardium of rats born and raised in hypoxic conditions

Effects of chronic hypoxia on capillary and myocyte growth were examined in rats born and raised in a low pressure chamber (equivalent of 3 500 m a.s.l.). The animals were sacrificed at the age of 3 months and their hearts were used to study right ventricular growth and vascularization. The results of our cytological and morphometric analysis suggest the persistence of capillary neogenesis in this particular model of cardiac hypertrophy. Under the optical microscope, we observed significant changes in capillary spatial patterns such as the presence of sinusoids and irregular capillary sprouts. This resulted in a significant shortening of the effective diffusion distance and in a slight decrease in the calculated diameter of the Krogh cylinder. Concomitant to the remodeling of the terminal capillary network, the right ventricular myocardium of hypoxic rats exhibited peculiar changes in myocyte cytology. The principal alteration consisted in the ectopic subsarcolemmal location of some of muscle cell nuclei which appeared enlarged and rounded, sometimes irregularly folded. At the E. M. level, they presented chromatine condensation, nucleolemmal folding and, occasionally, nuclear splitting. Irregular chromatin densifications at the equatorial position were also encountered but we never observed nucleolemmal dissolution or typical metaphase plaques which excludes the presence of mitotic division. Some of the marginalized nuclei were progressively excluded from original binucleate cells into small cytoplasmic processes that invaded the adjacent neoformed pericapillar spaces and gave rise to small well-organized cardiomyocytes. This apparent fragmentation of cardiomyocytes may evoke the description of the apoptotic process which is believed to be stimulated in hypoxic tissues. However, we could not confirm that myocyte fragmentation that we describe is followed by shrinkage necrosis or by any mobilization of adjacent resident cells. Nuclear exclusions into pericapillary myocyte sprouts may, therefore, reflect amitotic divisions of polyploid cardiomyocytes which contribute to the persistence of hyperplasic growth in right ventricular myocardium in hearts of rats exposed to chronic hypoxia during their early postnatal life. Par analogie with our data, it can be expected that an appropriate stimulation of angiogenesis in hearts of adult animals attenuates some of cytological and functional drawbacks that accompany hypertrophic cardiomyopathies of other etiologies.