Developmental changes in the human cardiac isomyosin distribution: An immunohistochemical study using monoclonal antibodies

Abstract

With monoclonal antibodies (Mab) specific for myosin heavy chain (MHC) isozymes, we have investigated the isomyosin content of atrial, ventricular and conductive fibers of 19 human fetuses (ranging from 14-36 weeks of gestation) and 3 newborns (2 days-2 weeks). In addition, the conduction system of 2 human adult hearts was studied. The fetal atrium is composed mostly of α-MHC during the first 23 weeks of gestation. β-MHC is already expressed as traces at 14 weeks of gestation, and its expression increases progressively until birth, resulting in a great augmentation in β-MHC. During this course, β-MHC always predominates in certain areas (the crista terminalis and the interatrial septum) but not in other areas (the auricles). Preceding birth, the fetal ventricle is composed mostly of β-MHC. From 14 weeks of gestation to birth, α-MHC is expressed in very rare fibers. Then, after birth, a large number of fibers simultaneously synthesize α-MHC. The AV node and His bundle system were labelled with anti-α and anti-β Mab in fetal, newborn, and adult hearts with a double gradient of distribution: spatial (a higher proportion of α-containing fibers in the AV node than in the distal portion of the bundle of branches) and temporal (a higher proportion of α-containing fibers at a given point of fetal development than in the adult heart). One of the twenty-five hearts studied had an isomyosin distribution pattern not accorded to its age. Interestingly, it was clinically diagnosed as having idiopathic hypertrophic cardiomyopathy.