Figure 2.

Early stages of heart development. (A) Ventral views of the developing mouse embryo. At E7.0, cardiac progenitors (red) have reached the head folds and, by E7.5, two cardiac lineages can be distinguished: the first heart field (FHF) (red) and the second heart field (SHF) (blue). At E8.0, the FHF progenitors merge to form the heart tube, which elongates at arterial and venous poles by the addition of progenitor cells from the second heart field (SHF). Between E8.0 and E9.0, the heart tube undergoes rightward looping. (B) Ventral view of the E9.5 heart, which consists of four anatomically distinct regions: atrium (At), atrioventricular canal (AVC), ventricle (V), and the outflow tract (OFT). (C) Longitudinal section depicting the prevalvular ECs. Two elongated cushions can be seen in the OFT, consisting of proximal (conal cushions, purple) and distal (truncal ridges, green) sections. The AVC has four cushions: right lateral (rlAVC), left lateral (llAVC), superior (sAVC), and inferior (iAVC). (Figure created from data adapted from Snarr et al. 2008.)