Fig. 3. Between E10.5 and E15.0, the expression of downstream targets and mediators of Wnt signaling correlates with sites of Wnt reporter activity. (A) At E11.5, Msx1 is robustly expressed throughout the mouse maxillary (mx), lateral (l) and median (m) nasal prominences, whereas the frontonasal (f) prominence lacks Msx1 expression (red arrow). (B) At E11.5, Msx2 is also expressed in the maxillary (mx), lateral (l) and median (m) nasal prominences but is absent from the frontonasal (f) prominence (red arrow). (C) At E11.5, Lef1 is expressed in the maxillary (mx) and, to a lesser extent, in the lateral nasal (l) prominences. Expression is largely absent from the frontonasal (f) prominence (red arrow). (D) Tcf4 expression is confined to domains within the lateral (l) and median nasal (m) prominences, whereas the frontonasal (f) lacks Tcf4 expression (red arrow). (E,F) Ventral view at E15.5 shows Msx1 and Msx2 expression limited to the maxillary (mx) prominences (dotted white line) and absent from frontonasal (f) prominence and the primary (1^o) palate. (G) At E13.5, Lef1 transcripts are detected in mesenchyme and ectoderm of the maxillary (mx) prominences, and at lower levels in frontonasal (f) mesenchyme. (H) Tcf4 transcripts are evident through the frontonasal (f) and maxillary (mx) mesenchyme. (I) At E10.5, Nmyc transcripts localize to maxillary (mx) mesenchyme and are largely absent from frontonasal (f) tissues (red dotted line). (J) At E13.5, Nmyc is expressed in all surface ectoderm and continues to be expressed in maxillary mesenchyme (mx) but is absent from frontonasal (f) mesenchyme (red dotted line). (K) At E15.5, Nmyc maintains its same general expression pattern (red dotted line). (L) At E10.5, PCNA immunostaining on transverse sections shows evidence of cell proliferation in frontonasal (f) ectoderm and maxillary (mx) mesenchyme. (M) Co-staining for BrdU and ß-gal shows that at E13.5, cell proliferation and reporter activity are colocalized to maxillary (mx) mesenchyme, whereas decreased levels of cell proliferation and reporter activity are detected in frontonasal (f) mesenchyme (dotted red line). Scale bars: 100 µm.