Atrial septation in the chick and rat: a review.

Septum primum of the chick and rat heart develops as a solid, crescentic fold on the inner atrial wall. In the chick this septum forms during days 3 and 4 of incubation. The same septum begins development during day 12 in the rat. In both species the septum grows rapidly toward the two endocardial cushions, decreasing the size of foramen primum in the process. A second foramen, foramen secundum, develops in the septum primum to maintain an interatrial shunt. Multiple foramina form in the middle and dorsal portions of the chick septum primum. These foramina continue to form until the middle of the second week of incubation. Prominent rounded cells are abundant on the chick septum in the region of foramina formation. These cells are no longer demonstrable when new foramina cease to be formed. At the time of hatching the cords of tissue separating foramina have thickened and fewer foramina exist. By day 2 after hatching the septum is an imperforate partition. The septum is seen to protrude into the left atrium, even after hatching. Foramen secundum forms very rapidly in the rat. Scanning electron microscopy indicates that it develops initially as multiple perforations which quickly coalesce into one aperture in the middle and ventral portions. The rat has a septum secundum which develops just to the right of septum primum. The valves of the sinus venosus project into the right atrium of the rat heart. Nodules are prominent features of the right valve early during the third week of gestation.