Atrial septation in the rat. II. An electron microscopic study.

The purpose of this investigation is to examine the process of atrial septation in the embryonic rat heart utilizing scanning (SEM) and transmission (TEM) electron microscopy. SEM shows septum primum, the first partition to develop, to extend from the dorsocranial wall of the primitive atrium by day 12.50. This begins the division of the common chamber into right and left portions. As septum primum grows foramen primum decreases in size and a second interatrial communication develops. Foramen secundum rapidly develops in the ventrocranial portion of septum primum during days 13.00 through 15.50. TEM studies of septum primum in the area of foramen secundum formation demonstrate that foramen formation is accomplished through transseptal communications established by cellular processes of rounded-up endocardial cells. These extensions protrude into the septal core isolating the septal tissue into small cords and the size of foramen secundum is increased. Areas of extremely dilated rough endoplasmic reticulum are prominent in the rounded-up endocardial cells and have a characteristic honeycomb appearance. No evidence of cell death is seen. A second septum develops to the right of septum primum and scanning electron micrographs show it to be a definitive partition by 16.50 days. At the fine structural level, elongated endocardial cells cover a center of myocytes containing many myofibrils, ribosomes and mitochondria.