Myocardial interatrial septum loses its epithelial organization by mesenchymal influence. Structural and ultrastructural study.

During atrial septation, the septum primum fuses with the atrioventricular endocardial cushions and myocardial-mesenchymal interactions occur. In order to evaluate the cellular events that take place during this particular interaction a structural, ultrastructural and histochemical study was performed. Our findings indicate that from the fourth day of development, the distal myocardium of the interatrial septum, which interacts with mesenchymal tissue, loses its appearance of an epithelial sheet and becomes a loosely organized tissue. The distal myocytes of the interatrial septum which get progressively separated show features of migratory cells, the final localization of which is the mesenchymal tissue of the adjacent endocardial cushions. These tissue changes involve basal membrane disruption, reduction in the number of desmosomes and intercalated discs with the subsequent appearance of large intercellular spaces between myocytes, myofibrillar disarrangement and acquisition by myocytes of a secretory phenotype characterized by numerous cytoplasmic vesicles. These events occur in a similar way in the atrioventricular canal, where a myocardial-mesenchymal interaction also occurs. In both regions the mesenchymal endocardial cushions and its associated extracellular matrix seem to direct the dissociation of the myocardial tissue and the subsequent migratory cellular behaviour of the interacting myocytes. This is an interesting, and little known, example of a cellular phenotypic transformation during cardiac development.