Fig. 4.

Model of the molecular pathway that controls asymmetric PE development. In the early gastrulating chicken embryo a functional midline is established that divides the left from the right side (red line). Endogenous H⁺/K⁺-ATPase-dependent difference in membrane voltage potential exists between the left (−) and right (+) sides of the primitive streak. In the midline, cells are fated to undergo apoptosis (green bar). At HH stages 4–6, asymmetric gene expression within Hensen's node is established leading to the induction of the Nodal-Pitx2 pathway on the left side, while FGF8 induces Snai1 on the right. The Nodal-Pitx2 pathway on the left side has no effect on PE-specific marker gene expression. In contrast, Snai1 on the right side is required for Tbx18 and Wt1 expression.