Figure 1. WNT signaling regulates left-right (LR) determination. (A through C) Scanning electron microscopy micrographs of ventral views of developing hearts on embryonic day 9.5 (E9.5). (A) The wild-type outflow tract (ot) loops to the right. Wnt3a–/– hearts display normal looping (B), inverted looping (situs inversus) (C), or loops that remain in the midline (situs ambiguus, not shown). Ventral-posterior views of E8.2 wild-type (D) and mutant (E) embryos processed for two-color whole-mount in situ hybridization showing Nodal (orange) and Lefty1 (purple) expression in the node (n), and overlapping Nodal (orange) and Lefty2 expression (purple) in the left lateral plate mesoderm (lpm). Note the absence of staining in the Wnt3a–/– lpm and the aberrant expression in the node. (F) WNT3A regulates LR determination and segmentation via the Delta/Notch pathway. The diagram depicts a perspective similar to that shown in (D) and (E). Wnt3a is expressed in the primitive streak and posterior node (red stippling) where it directly activates Dll1 expression in the mesoderm surrounding the node (blue arrows). In turn, DLL1 directly activates Nodal (red N) expression at the node/mesoderm boundary to set up the LR axis, and regulates segmentation (dashed arrows) in the presomitic mesoderm (psm). How LR asymmetric gene expression (green gradient) is established in the node is poorly understood, but involves node cilia. L, left; R, right; A, anterior; P, posterior; lv, left ventricle; s, somite; L1, Lefty1; L2, Lefty2.