MYOCARDIAL INTERATRIAL SEPTUM LOSES ITS EPITHELIAL ORGANIZATION BY MESENCHYMAL INFLUENCE - STRUCTURAL AND ULTRASTRUCTURAL-STUDY

During atrial septation, the septum primum fuses with the atrioventric ular endocardial cushions and myocardial-mesenchymal interactions occu r. In order to evaluate the cellular events that take place during thi s particular interaction a structural, ultrastructural and histochemic al study was performed. Our findings indicate that from the fourth day of development, the distal myocardium of the interatrial septum, whic h interacts with mesenchymal tissue, loses its appearance of an epithe lial sheet and becomes a loosely organized tissue. The distal myocytes of the interatrial septum which get progressively separated show feat ures of migratory cells, the final localization of which is the mesenc hymal tissue of the adjacent endocardial cushions. These tissue change s involve basal membrane disruption, reduction in the number of desmos omes and intercalated discs with the subsequent appearance of large in tercellular spaces between myocytes, myofibrillar disarrangement and a cquisition by myocytes of a secretory phenotype characterized by numer ous cytoplasmic vesicles. These events occur in a similar way in the a trioventricular 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, ex ample of a cellular phenotypic transformation during cardiac developme nt.