New embryological evidence for the formation of quadricuspid aortic valves in the Syrian hamster (Mesocricetus auratus).

A Syrian hamster embryo, aged 11 days and 4 h post-coitus, had a developing quadricuspid aortic valve. The septation of the cardiac outflow tract was confined to the distal part of the conotruncus. There was a conspicuous recess in the anlage that normally gives rise to the left aortic valve cushion. Globular endothelial cells arranged in several layers were present at the luminal side of the recess. The present findings support the hypothesis that in the Syrian hamster, quadricuspid aortic valves result from the division of one of the three mesenchymal anlagen that give rise to normal aortic valves. In addition, they indicate that the division of the anlage is due to the invagination of the endothelial layer that covers its luminal side. The invagination of the endothelium starts at a very early stage of the valvulogenesis, namely, when the conotruncal ridges begin to fuse at the distal portion of the embryonic cardiac outflow tract.