In situ morphology of the heart and great vessels in fetal and newborn rats.

Morphology of the cardiac chambers and great vessels of fetal and neonatal rats was studied using the whole body freezing technique and by sectioning through the short axis of the heart with a freezing microtome. Compared to the fetal heart, the neonatal heart showed rapid change 2 to 8 days after birth. The ventricular sinus septum was straight in the fetus and became concave to the left ventricle after birth. The right ventricular wall was as thick as the left in the fetus and became thinner rapidly after birth. At the same time, the right ventricular cavity dilated. The right and left pulmonary arteries and pulmonary veins were small in the fetus and enlarged soon after birth. At the same time, the foramen ovale was closed and the diameters of the inferior vena cava and descending aorta were diminished. One-half-mm thick sections were cut serially and then photographed. Ventricular volumes and masses were calculated from summation of the areas of each chambers. Left ventricular mass per body weight increased rapidly after birth, whereas right ventricular mass per body weight remained constant from 0 to 8 days after birth.