Cell shape and cytoskeletal organization of the endothelial cells of the semilunar heart valves in the developing chick.

The composition and arrangement of the cytoskeletal elements of the endothelium of the semilunar valves have been studied in the embryonic chick heart during the stages 30 to 38. Microtubules, vimentin intermediate filaments and actin microfilaments were constant components of the valvular endothelial cells in the studied stages. Scanning electron microscopy after Triton-X-100 extraction revealed significant differences in the tridimensional arrangement of the cytoskeleton in the course of valve development. In the ventricular face of the cusps the cytoskeletal elements displayed a progressive longitudinal alignment, while in the arterial face of the cusps the cytoskeleton maintained the appearance of a network. Transmission electron microscopy revealed that these differences were especially prominent for vimentin intermediate filaments, although a similar tendency was also observed for microtubules. Microfilaments were scarce in the endothelial cells of both faces of the cusps, and the stress fibers typical of the endothelial cells of the adult valves were not observed in the embryonic material. The significance of these results in valve morphogenesis and histogenesis and a possible linkage with differences in the local characteristics of the blood flow are discussed.