Figure 19.

The images are from embryonic mice in which the gene for Tbx1 has been knocked out. All these mice develop with common arterial trunks. The images are from embryos killed on the 13th day of development. The left panel (A) shows the common trunk, feeding the systemic and pulmonary arteries, with no formation of the aortopulmonary septum. It also shows failure of fusion of the outflow cushions. The right panel, from a different embryo at the same stage of development, shows a cross section through the intermediate part of the outflow tract. Both the intercalated cushions are seen, together with the distal ends of the unfused major outflow cushions. This template provides the primordiums for the formation of a common truncal valve with four leaflets. Note that the cushions are contained within the turret of myocardium that surrounds the intermediate part of the outflow tract.