Quantitative structural analysis of the myocardium during physiologic growth and induced cardiac hypertrophy: a...

The quantitative structural properties of the ventricular myocardium during postnatal physiologic growth are compared with those accompanying an increased load in the adult rat heart to determine whether induced cardiac hypertrophy is a pathologic condition or simply a form of well compensated accelerated growth. The expansion of the ventricular myocardium during maturation shows a remarkable degree of well balanced compensatory response, because the capillary microvasculature, parenchymal cells and subcellular components of myocytes all grow in proportion to the increase in cardiac mass. In contrast, the increases in myocyte diameter and length caused by pressure hypertrophy, volume hypertrophy and infarction-induced hypertrophy are consistent with concentric, eccentric and a combination of concentric and eccentric hypertrophic growth of the whole ventricle, respectively. These cellular shape changes may represent a compensatory response of the myocardium at the cellular level of organization that tends to minimize the effects of an increased pressure or volume load, or both, on the heart. Cardiac hypertrophy, however, may also show alterations affecting capillary luminal volume and surface and the mitochondrial to myofibril volume ratio, which indicate an inadequate growth adaptation of the component structures responsible for tissue oxygenation and energy production. Thus, hypertrophy of the adult heart differs from that during physiologic growth, and the hypertrophied myocardium may exhibit structural abnormalities that can be expected to increase its vulnerability to ischemia.