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

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.