Fig. 5.

(Overleaf) The heart-forming fusion model with respect to different types of cardia bifida. A–C: A transversal section through an embryo at the level of fusion of the heart fields. A′–C′: A ventral view (mesoderm only) of how the zipper complex is formed and fuses in a bi-directional fashion in corresponding embryos (anterior/cephalic vs. posterior/caudal; arrows). A,A′: One- to foursomite stage. B,B′: Five- to six-somite stage. C,C′: Seven-to tensomite stage. A: The mesoderm separates to form the coelom (arrowhead). Later, the mesoderm folds (twisted arrows in A′), and the previously outer margins (blue) meet to form the ventral fusion line, which originates at the level of the prospective right ventricle (area shaded with green horizontal lines in C’), at the same time the pericardial coelom is formed too (twoarrowheads). Fusion proceeds in both directions (↕), which allows for the caudal incorporation of the apical trabecular portion of the left ventricle (white area) and the cephalic addition of the prospective outlet (yellow area). D: Remnants of the ventral fusion line are located at the caudal and cranial aspects in a normal 16-somite embryo (open stars). If the complex fusion does not form, then a complete cardia bifida occurs (not shown). E: Failure in cranial extension (↓) would produce a biconal heart. F: Failure in the caudal extension (↑) would predict a posterior cardiac bifida. G: An embryo painted with green and red mitotracker showed defective fusion in both cranial and caudal directions and produce anterior and posterior bifide heart (H). This embryo did not develop beyond the 10 somite stage. Prospective outlet tissue (yellow), apical trabecular region of the right (horizontal hatch), left ventricle and AV canal (white), sinoatrial tissue (perpendicular hatch), and ventral fusion line (blue). The green represents the MF20 staining during several developmental stages (B′, C′, D, E, and F).