Commitment, differentiation, and diversification of avian cardiac progenitor cells

Three-dimensional visualization of connexin 43 on the human cardiomyocytes

Gap junctions created by a family of connexin proteins play an important role in the development of human heart. It has been previously shown that the abnormalities of right ventricular outflow tract are related to an altered level of expression of connexin 43. The right ventricular outflow tract narrowing, stenosis, or atresia of the main pulmonary artery and hypertrophy of the right ventricle are observed in tetralogy of Fallot. The aim of the current study was to determine the distribution of connexin 43 on the surface of human cardiomyocytes obtained during reparative surgery for tetralogy of Fallot. Connexin 43 distribution in these cells was compared with distribution of connexin 43 in cardiomyocytes obtained from patients without right ventricular outflow tract pathology. Cardiomyocytes isolated from tissue biopsy were cultured on collagen substratum, fixed with paraformaldehyde, and incubated with goat antihuman connexin 43 antibodies and secondary donkey antigoat antibodies conjugated with fluorescent indocarbocyanine. Z-series of optical sections were recorded using a laser scanning confocal microscope. Three-dimensional data stacks were visualized using volume-rendering techniques. Images of connexin 43 fluorescence revealed a pattern of three-dimensional distribution of connexin on the surface of an individual cardiomyocyte. Cardiomyocytes from tetralogy of Fallot and hearts with normal right ventricular outflow tract differ in the organization of connexin 43. Cardiomyocytes from tetralogy of Fallot hearts revealed disturbed distribution of connexin 43. The protein is located irregularly on the entire surface of the cell. In the controls, connexin 43 can be visualized within the intercalated disks only. These disturbances may influence heart maturation, cause hypertrophy of the right ventricle, and induce severe arrhythmias in children with tetralogy of Fallot.

Developmental remodeling and shortening of the cardiac outflow tract involves myocyte programmed cell death

The embryonic outflow tract is a simple tubular structure that connects the single primitive ventricle with the aortic sac and aortic arch arteries. This structure undergoes a complex sequence of morphogenetic processes to become the portion of the heart that aligns the right and left ventricles with the pulmonary artery and aorta. Abnormalities of the outflow tract are involved in many clinically significant congenital cardiac defects; however, the cellular and molecular processes governing the development of this important structure are incompletely understood. Histologic and tissue-tagging studies indicate that the outflow tract tissues compact and are incorporated predominantly into a region of the right ventricle. The hypothesis tested in the current study was that cell death or apoptosis in the muscular portion of the outflow tract is an important cellular mechanism for outflow tract shortening. The tubular outflow tract myocardium was specifically marked by infecting myocytes of the chicken embryo heart with a recombinant replication-defective adenovirus expressing beta-galactosidase (beta-gal) under the control of the cytomegalovirus promoter. Histochemical detection of the beta -gal-labeled outflow tract myocytes revealed that the tubular structure shortened to become a compact ring at the level of the pulmonic infundibulum over several days of development (stages 25–32, embryonic days 4–8). The appearance of apoptotic cardiomyocytes was correlated with OFT shortening by two histologic assays, TUNEL labeling of DNA fragments and AnnexinV binding. The rise and fall in the number of apoptotic myocytes detected by histologic analyses paralleled the change in activity levels of Caspase-3, a protease in the apoptotic cascade, measured in outflow tract homogenates. These results suggest that the elimination of myocytes by programmed cell death is one mechanism by which the outflow tract myocardium remodels to form the proper connection between the ventricular chambers and the appropriate arterial trunks.