Figure 1.

Early events in heart tube formation. (A) The forming human heart in a ventral view and transverse section at stage 9 showing the heart tube open dorsally to foregut endoderm. (B) Quantitative 3D reconstruction of Ki67-positive cells at stage 10 showing the paucity of proliferating cells in the myocardium compared with the dorsal pericardial wall. Note the local increase in proliferation (arrowheads) representing the initiation of ballooning morphogenesis. (From Sizarov et al. 2011; reproduced, with permission.)