Development of mouse semilunar valves.

The development of the semilunar valves of the great arteries of the mouse is studied by light and scanning electron microscopy. The earliest anlage of the valves is observed at day 11.5 of gestation, as three pairs of tubercles protruding towards the lumen of the truncus arteriosus. These early cusps consist of a core of mesenchymal tissue covered by the endocardium. From day 12.5 to 14 a process of excavation takes place in the arterial face of the cusps resulting in the achievement of their final shape. Our observations suggest that the excavation process is produced by a selective growth of the free edges of the cusps. Numerous pieces of evidence are reported suggesting that the selective growth of the cusps is due to an inductive interaction between the endocardium of the arterial face of the cusps and the underlying mesenchyme. The histogenesis of the cusps takes place very late in development and possibly continues into the postnatal period. The results are compared with studies made in other vertebrates.