Morphogenesis of the truncus arteriosus of the chick embryo heart: the formation and migration of mesenchymal tissue.

The appearance and migration of mesenchymal cushion tissue within the truncus arteriosus of the normal 2.5 to 6-day chick embryo heart was surveyed systemically with the light microscope. Series of cross-sections taken from replicate hearts at successive developmental stages allowed comparison of the following qualitative and quantitative aspects of early truncal morphogenesis. Mesenchyme within the truncus was derived from two distinct sources. The first mesenchyme appeared to migrate caudally into the cardiac jelly of the distal truncus from the nearby aortic arch region, coincident with slowing of the anterior elongation of the heart tube (Hamburger-Hamilton Stage 17-18). A second, separate mesenchymal population, derived from endocardium, began to fill the conus and proximal truncus in a radial direction, coicident with expansion of the bulbs cordis (Stage 12-19). The measured kinetics of relative cell numbers, distributions, and mitotic indices suggest substantial contributions from both sources. By Stage 26, the conotruncal region was filled with mesenchyme, which then condensed to form the anlagen of three future structures: the semilunar valves, the aorticopulmonary septum, and the tunica media of the great arteries.