Atrial septation: I. Scanning electron microscopy in the chick

Chick hearts were prepared for scanning electron microscopy by standard methods, the purpose being to investigate the surface morphology of the developing atrial septal region. By Day 3, the atrial septum primum appeared as a sickle-shaped structure. During Day 4, the first representation of the foramina secunda occurred in the mid-dorsal portion of the septum. During Day 5, the septum primum fused with the atrioventricular endocardial cushions, thereby occluding the foramen primum. From Days 5 through 8, the secondary perforations (foramina secunda) multiplied and increased in size. The endocardial-covered cords of cells comprising the septum thickened from Days 9 through 15. This resulted in a marked reduction in the dimensions of the perforations from Day 16 to hatching. The atrial septum at hatching occasionally contained a small single orifice. At 3 days posthatching, the atrial septum was a solid sheet covered with flattened endocardial cells. All interatrial communications were occluded. During Days 5 through 9, two distinct cell types became apparent on the endocardial-covered cords. Simultaneously, fenestrations were observed on the cords surrounding the foramina secunda and on the ventral portion of the atrial septum. The integral role which the fenestrations and cellular types play in the development of the formina secunda is discussed.