Fig. 2. Wnt signaling controls LR asymmetric gene expression. (A-P) Whole-mount in situ hybridization analysis of Nodal expression (see text for details). Headfold-stage wild-type (A,E), and Wnt3a^-/- (B,F) embryos express Nodal (purple) in the node. Two-color whole-mount in situ hybridization showing Nodal (orange), and Lefty1 and Lefty2 expression (purple) in wild-type (C,G) and mutant (D,H) three-somite stage embryos. (G,H) High-power ventral views of the nodes of the wild-type and mutant embryos depicted in C,D. Nodal expression in wild-type four-somite (I,M) and similarly staged Wnt3a^-/- (J,N) embryos. Arrows in N indicate bilateral Nodal expression in the mutant posterior LPM. Nodal was not expressed in the wild-type LPM after the six- to seven-somite stage (K,O), but was bilaterally expressed in the mutant LPM (L,P). (Q-S) Similar abnormal expression patterns were observed for Lefty2 in the seven-somite stage mutants (right embryos in Q and R, compare with the four-somite wild-type embryos on the left), and for Pitx2 expression in six-somite mutants (S). (T,U) Two-color whole-mount in situ hybridization showing Nodal (purple) and Wnt3a (orange) expression in four-somite wild-type (T) and Axin^Tg1/Tg1 (U) littermates. (V,W) Lefty1/2 (purple) and Nodal (orange) expression in two-somite wild-type (V) and Axin^Tg1/Tg1 (W) littermates. Asterisks indicate ectopic bilateral expression of Nodal (U) and Lefty1/2 (W) in anterior domains. (A-D,I-L,Q,T-W) lateral views; (E-H,M,N,S) ventral posterior views; (O,P,R) anterior views; arrows indicate the LPM; arrowheads indicate the anterior limit of LPM expression. ps, primitive streak; n, node.