Figure 4.

Contributions of the cushions and ICVS to the arterial valve primordia. Histological sections of mouse hearts stained with H&E. (A) At E10.5 the outflow is unseptated. The cushions (arrows) are forming and can be seen to extend along the length of the outflow tract in transverse view and to be circumferential in both the distal and proximal regions, in frontal view. The ICVS cannot be seen in transverse sections at this stage, but in frontal sections can be seen distally as cell dense regions within the outflow wall (arrowheads). (B) By E11.5, the cushions are more cell dense and in transverse sections the valve forming part (red bar) can be seen to be distinct from the proximal part of the cushions that will form the sub-pulmonary infundibulum in transverse sections. The ICVS (arrowhead) can now be seen adjacent to the valve-forming parts of the cushions. In frontal sections the main cushions (arrows) are coming together towards the midline of the outflow tract in the distal region. The ICVS (arrowheads) have expanded but are still continuous with the outflow wall. More proximally, the cushions (arrows) are still widely separated. (C) By E12.5 the outflow tract has septated and the primordia of all three leaflets are apparent in transverse and frontal views. The proximal region of the outflow tract remains unseptated at this time point, although the cushions (arrows) are beginning to come together. A = anterior leaflet; L=left leaflet; P = posterior leaflet; R = right leaflet.