Figure 4. Scanning electron micrographs of E10.5 and 11.5 mouse embryos.

A) (E10.5) and B-D) (E11.5) show the junction of the OFT and the mediastinal wall, viewed inferiorly, with the transected foregut and bilateral cardinal veins visible beneath. Note the progressive asymmetric bulging of the left and right 6th aortic arches (labelled 6 in B,C,D,E) into the pericardial cavity. This, together with excavation at the pericardial border, gives the impression that the OFT is being twisted anticlockwise in this ventral view. In D), the OFT has been transected in the plane indicated by the white line in C. This shows that the lumen at the site of the cut is made up of a large right-sided cranial portion that will be the intrapericadial aorta (ipa), and a small left-sided caudal portion that will be the pulmonary trunk (ipp). The double-headed white arrow shows the oblique orientation of the AP foramen and the protrusion. E) shows an OFT that has been sectioned longitudionally (transverse to the embryo, at the level shown by the white line in D) and is viewed superiorly. Note the narrowness of the junction (arrows) between OFT and body. The septal cushion (SC) is visible in the inferior portion of the OFT. F) shows a late E11.5 junction of the OFT and the mediastinal wall, viewed superiorly, showing the pronounced remolding between the ipa and ipp.

aa – aortic arch.