FIG. 4.

Temporal changes represented diagrammatically along the anterior to posterior (A/P) axis in the cardiac progenitor cells of the primary heart fields leading to the differentiated phenotype. HH stage 3/4−: Mesenchymal cells in the mesoderm (light grey) remain pluripotential. These cells are sensitive to Ca⁺⁺ blockade. HH stage 4: Cells in the anterior most regions of the field are committed and begin the cell sorting process. The sorting cells are now refractory to Ca⁺⁺ blockade (asterisks). More posteriorly cells are being specified toward the cardiac lineage by expressing β-catenin and N-cadherin to define the future dorsal boundary of the cardiac compartment [21,22,35]. Cell sorting has not yet been initiated and cells remain sensitive to Ca⁺⁺ blockade. HH stage 5/6: Cells in the anterior most regions have now sorted out to form the epithelial, polarized, differentiating cardiac compartment, and cells just more posterior are initiating steps toward sorting (asterisks), leading to pericardial coelom formation (black). These cells are refractory to Ca⁺⁺ blockade. More posterior cells are just beginning to commit and are becoming specified along an A/P gradient. These posterior cells remain sensitive to the blockade. HH stage 7: Most of the primary heart field is now becoming committed anteriorly and cells are in the process of cell sorting, except for the cells in the most posterior part of the heart field. Bottom row shows representative embryos for each stage depicted.