Changes in myocardial collagen in normal development and after beta blockade.

During development the total mass of collagen increases to the same degree in each ventricle independently of the mass of muscle. At birth the concentration of collagen is the same in each ventricle. Thereafter a greater quantity of muscle is developed in the left ventricle than in the right ventricle, so that the concentration of collagen in the right ventricle comes to exceed that in the left. In the mature rabbit the concentration of collagen is higher in the atria than in the ventricles, higher in the right ventricle than in the left ventricle, and higher in the epicardium than in the endocardium. Beta blockade has no effect on the content of myocardial collagen.