Fig. 5. Wnt8 is required to antagonize Tcf3-mediated repression but not to activate caudal genes. (A-C) Animal pole views, dorsal towards the right. (A) At shield stage, hdl is expressed in the neurectoderm where Wnt signaling is low. (B) By contrast, lef1 is expressed in the ventrolateral mesoderm where Wnt signaling is high. (C) Reduction of Tcf3 function results in expanded lef1 expression. (D,E) Wild-type pax6 expression at one to three somites. Arrowheads in E define the caudal limit of pax6 in the forebrain and rostral limit of pax6 in the hindbrain. (F) Wild-type pax2.1 expression at one to three somites. (G-I) In the absence of hdl function, pax6 is shifted rostrally (arrowheads) and rostral expression is reduced, whereas pax2.1 is expanded rostrally. Changes in the intensity of caudal pax6 expression did not correlate with changes in other caudally expressed genes (see Fig. 4). (J-L) In the absence of both hdl and wnt8 function, pax6 is still reduced and shifted rostrally (arrowheads) and pax2.1 is still expanded. The neural plate is wider in these embryos because of the role of wnt8 in dorsoventral patterning. (M-O) Loss of wnt8 function results in caudal expansion of pax6 (arrowheads) and a caudal shift in pax2.1.

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