Fig. 2. The primordia of the MHB and the hindbrain affected in spg embryos. (A-B',E-F', I-J',N-N',Q,Q',U-U') Dorsal views; embryos in the remaining pictures are shown from lateral view. Gene expression, stages and genotypes are noted. Red arrowheads indicate expression of genes at the MHB throughout. (A-D') In spg embryos, pax2.1 shows reduced expression at the MHB from its onset onwards (A',B'), is lost during midsomitogenesis (C') and re-expressed as a dorsal patch after 24 hpf. Expression of pax2.1 within the otic placode (B-D') is not affected in mutant embryos. (E-H') wnt1 is normally expressed at its onset at 80% of epiboly (not shown), but becomes subsequently downregulated in mutant (E') at the time when pax2.1 is initiated. During somitogenesis (F-G'), the expression of wnt1 at the MHB (arrowhead) and within rhombomeres is downregulated in spg embryos. At pharyngula stages, wnt1 expression is continued within a dorsal patch at the MHB. (H,H') The midsagittal expression in the diencephalon seems unaffected, but MHB expression is reduced to a dorsal patch in mutant embryos. (I-I') fgf8 expression, like that of wnt1, is not affected in spg embryos at its onset of expression at the MHB (not shown), but soon becomes downregulated at around 90% of epiboly. (J) fgf8 expression caudally continues in r1, r2 and r4 in wild-type embryos. (J') In mutant embryos, fgf8 expression is strongly reduced within r1 and abolished within r2 and r4. During somitogenesis, fgf8 expression is completely lost from the MHB but, like pax2.1 and wnt1, recovers at a dorsal patch at the MHB. (M-P') spry4 is not properly initiated in spg embryos. At the four-somite stage, spry4 is strongly reduced at the MHB and in r1, r2 and r4 (N'). MHB expression of spry4 during somitogenesis and pharyngula stages follows the same mode as fgf8 and pax2.1. (Q,Q') en2 is normally initiated at the MHB at the end of gastrulation. In spg mutant embryos, en2 is downregulated from its initiation of expression. (R) en2 is expressed in the prospective tectum in a graded fashion during somitogenesis but is strongly reduced in spg embryos (R'). (S) en3 is encompassed within the en2 domain at the tectum in wild-type embryos. (S') In spg embryos, en3 is downregulated in a similar fashion as en2. (T,T') half sides of transverse sections through the her5 positive domain at the spatial level of the future MHB; arrows point to the neuroectoderm. (T) her5 is normally initiated within the neuroectoderm around 70% of epiboly, overlying mesendodermal expression. (T') her5 is not properly initiated in spg embryos. (U,U') her5 expression at the MHB is reduced in mutant embryos at the end of gastrulation.