Atrial septation in the rat. I. A light microscopic and histochemical study.

Light microscopy and histochemistry consisting of alcian blue staining were utilized to study atrial septation in the albino rat heart. Light microscopy shows septum primum to develop from the dorsocranial wall of the primitive atrium by day 12.50. Foramen secundum rapidly develops in the ventrocranial portion of septum primum during day 13.00. A second septum appears to the right of septum primum in the rat as the foramen secundum increases in size. The first evidence of septum secundum can be noted on histologic preparations as a small rounded bulge extending into the right atrium from the ventrocranial wall. This occurs at day 14.50. This septum continues to enlarge but does not fuse with the endocardial cushions as does septum primum. Alcian blue staining shows the presence of glycosaminoglycans (GAGs) concentrated heavily in the leading edge of the 12.50 day septum primum. GAGs are also present during foramen secundum formation in areas where endocardial cells aggregate. As development progresses, GAGs diminish in all areas of septum primum. Of particular interest is an area of intense alcian blue staining in the wall of the right atrium ventral and caudal to septum secundum development. This appears in histologic preparations of 13.50 and 14.50 day rat hearts.