Scanning and light microscope studies of the development of the chick embryo semilunar heart valves.

The development of the semilunar valves of the great arteries was studied by light and scanning electron microscopy in the chick embryo. The results show that three distinct developmental periods can be distinguished. The formation of the anlage of the valves takes place in the first period (stages 26--29). These early anlage consist of three pyramidal shaped cusps formed by a core of loosely packed mesenchymal cells covered by a flattened endothelium. In the second period (stages 30--35) the cusps undergo excavation on their distal face. Morphololgical evidence is reported suggesting that this excavation process is produced by an initial solid ingrowth of the endothelium of the arterial face of the cusps which is immediately luminated by detachment of cells towards the bloodstream and by cell death. The histogenesis of the valves takes place in the third period (from stage 36 until hatching). It was observed that during this period some myocardial cells of the outflow tracts of the ventricles invade the valvular tissue and that in the upper part of the cusps a prominent fibrous layer is formed.