Morphometric study of early postnatal development in the left and right ventricular myocardium of the rat. I. Hypertrophy, hyperplasia, and binucleation of myocytes.

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Abstract
The absolute and differential growths of the myocyte populations in the left (L) and right (R) ventricular myocardium were reviewed morphometrically from 1-5 days and from 5-11 days after birth. From 1-11 days hypertrophy of the average myocyte in the ventricles was (L) 2.7- and (R) 2.4-fold and myocyte proliferation was (L) 2.0- and (R) 1.2-fold. Mean cell volume, cell length and percent binucleation of cardiac myocytes were similar in both ventricles at 1, 5 and 11 days of age. During this period average myocyte length increased 2-fold (12 sarcomere lengths) and the percentage of binucleate myocytes increased approximately from 2.7 to 17 to 48%. Myocyte hypertrophy from 1 to 5 days resulted mainly from an increase in the volume of cytoplasm per nucleus and from 5 to 11 days from the accumulation of binucleate cells. No differences were observed in the characteristics of epicardial and endocardial myocytes in either ventricle up to 11 days of age. The 61% greater proliferation of myocytes in the left ventricle was the principal basis for the development of a 2-fold difference in ventricular weight gains: (L) 6.2- and (R) 3.4-fold. No increase in right ventricular midwall thickness was observed in contrast to a 2.7-fold increase of the left ventricle. Due to the circulatory changes occurring shortly after birth, right ventricular growth is analogous to eccentric hypertrophy; left ventricular growth represents a combination of eccentric and concentric hypertrophy.