Pulmonary vestibule and the aortic infundibulum develop from different embryonic components.

(A, B). Adjacent cephalo-caudal histological sections of the embryonic heart at stage 28HH. Observe a wider opening zone at the originally left-dorsal myocardial conal wall (*). The voluminous mesenchymal structure separating the incipient right (1) and left (2) atrioventricular orifices corresponding to the ventro-superior and the dorso-inferior cushions of the AV canal during the fusion process (3). (A’, B’) Skeletal muscle myosin immunolocalization (green) corroborating the left-dorsal myocardial conal wall loss of continuity (*). (C) Stage 30HH. Visualization of the well-developed dextrodorsal (4) and sinistroventral (5) crests within the conus (C). The incipient mural leaflet of the left AV valve (6) is also evident. Note the elongated canal (7) bordered by the dorsal surface of the conal crest (4 and 5) and the right surface of the incipient anteroseptal leaflet of the left AV valve (8). (D, E). Stage 31 and 32HH. Mature heart showing the fused conal crests forming the immature supraventricular crest (9). The single conal conduct corresponds to the pulmonary infundibulum (PI). The aortic vestibule (AoV) behind the developing anteroseptal leaflet of the left AV valve (8) is well developed. E’. Amplification of the red box in E. Observe traces from the label as some fine charcoal granules immersed in the myocardium of the pulmonary infundibulum (40X). Abbreviations: RAVV = Right atrioventricular valve; LAVV = Left atrioventricular valve.