Figure 4. Multiple steps of cardiac development require PcG/TrxG function.

Precardiac mesoderm give rise to cardiac progenitors in the first heart field (FHF) and second heart field (SHF, also known as anterior heart field or AHF). The TrxG protein Baf60c likely regulates the induction of cardiac fate. Cells in the FHF form the linear heart tube, which gives rise to the bulk of the left ventricle (LV) and also serves as a scaffold for subsequent heart growth. As the heart tube loops, cells in the SHF migrate to join the linear heart tube and give rise to the outflow tract (O), right ventricle (RV) and atria (A). Both Baf60c and the PcG protein Phc1 have been shown to regulate this early phase of cardiac development. The formation of the chambered heart from the looped heart involves a number of morphogenic processes such as trabeculation, proliferation and septation. Multiple PcG and TrxG proteins, including Brg1, Baf60c, Ezh2, Eed and Jmj, have been shown to regulate these processes. The dashed line between Jmj and Ezh2/Eed represents possible functional interaction. In addition to the FHF and SHF, cells from cardiac neural crest and proepicardium also contribute to the heart (not diagrammed).