Atrial septation: II. Formation of the foramina secunda in the chick

Chick hearts were prepared for light microscopy and transmission electron microscopy by conventional methods, the purpose being to investigate the developing foramina secunda in the atrial septum. As the growing septum approaches and fuses with the endocardial cushions, small perforations (foramina secunda) are formed in the middorsal portion of the septum. During Days 5 and 6 the number and size of these foramina increase significantly. Formation of foramina creates thin cords of endocardium-covered tissue. Two varieties of endocardial cells are associated with the atrial septum during this period. The endocardial cells on the surface of the septum, some distance from the foramina secunda, are flattened and possess long, attenuated processes. Conversely, the endocardium in the rough portion of the septum near the foramina is characterized by the presence of numerous rounded cells which project notably into the atrial chambers. These rounded endocardial cells resemble phagocytic cells and possess thin cytoplasmic processes which extend into the septal core, gradually separating the cells of the core, resulting in an isolation of a portion of septal tissue. Once a transseptal communication is established the cytoplasmic processes separate, creating a new foramen secundum. The rounded endocardial cells are also responsible for maintaining the integrity of the endocardium-covered septum throughout the remodeling process. The origin of the rounded cells is unknown at this time. The most likely sources are differentiation from existing endocardial cells or circulating phagocytes.