Fig. 5. The hindbrain primordium is affected in spg embryos. Embryos are photographed from the dorsal side, with the exception of D,D' (transverse sections at the level indicated by an arrow in C,C') and E, E' (lateral views). Dorsal is upwards in D,D'. Anterior is towards the top in A-C',F,F',R,R',S,S'; anterior is to the left in the remaining pictures. Embryos are at the tailbud stage unless indicated differently. (A) gbx1 expression is strictly posteriorly adjacent to the MHB domain of pax2.1 in wild-type embryos during gastrulation. (A') In spg embryos, pax2.1 and gbx1 are expressed in the same mutually exclusive fashion as seen in wild-type embryos at the end of gastrulation. However, in mutant embryos pax2.1 expression is reduced at the MHB. (B) In wild-type embryos, otx2 expression partially overlaps pax2.1 expression at the MHB at the end of gastrulation. (B') spg embryos show a proper spatial relationship of otx1 and pax2.1 at the prospective MHB at the end of gastrulation. (C,D) In wild-type embryos, gbx2 becomes activated at around 90% of epiboly within the neural ectoderm, shortly after onset in the underlying mesendoderm. (C',D') In mutant embryos, the mesendodermal domain of gbx2 is initiated normally (red arrowhead in D') but the neurectodermal domain of gbx2 is not initiated. Two longitudinal stripes in the non-neural ectoderm are unaffected (black arrow in D'). (E,E') In contrast to spg mutant embryos, gbx2 is lost in both the mesendodermal and the neuroectodermal germ layer in ace mutant embryos. (F) The hindbrain domain of fkd3 is lost in mutant embryos (F'). (G) In wild-type embryos, krox20 stains r3 and r5, and six3 is expressed within the prosencephalon, including the prospective eye field. (G') In mutant embryos, six3 seems not affected but krox20 is strongly reduced. (H) ephA4 is expressed in wild-type embryos within the prosencephalon and the rhombencephalon, in particular within rhombomeres 1, 3 and 5. (H') Rhombomeric expression of ephA4 is strongly affected and the prosencephalic domain shows massive posterior expansion. (I) wnt8b is normally expressed within the diencephalon, at the MHB and within rhombomeres 1, 3 and 5. (I') In spg embryos, MHB expression of wnt8b is strongly reduced (arrowhead) and rhombomeric expression is strongly downregulated; in particular, r1 cannot be discriminated from and possibly fuses with the MHB domain. (J) Double in situ staining for hoxb1a, expressed in r4, and hoxb4a, expressed within the spinal cord with an anterior limit at the border between r6 and 7 in wild-type embryos. The bracket indicates the gap between r4 and 7. (J') In mutant embryos, the gap between r4 and r7, indicated by the bracket, is strongly reduced. (K,K',L) pou2 expression becomes refined during early somitogenesis within distinct bilateral clusters, according to r2 and r4, and to a patch of expression at the posterior border of the MHB. (L') In early somitogenesis, embryos of the allele spge713 show strongly reduced rhombomeric expression of pou2, whereas in embryos carrying the insertional allele spghi349, pou2 expression is totally abolished (L''). (M) val is normally expressed within r5 and 6. (M') In mutant embryos, val expression is nearly abolished in r5 but the expression in r6 is not affected. val is also expressed within precursor cells of the neural crest (indicated by arrows in M,M'), which is not affected in mutant embryos. (N) fkd3 is expressed at inter-rhombomeric borders at late somitogenesis stages in wild-type embryos. (N') In spg embryos, inter-rhombomeric expression is strongly reduced. (O) zath1 is normally expressed at the prospective cerebellum and along the dorsal rim of the fourth ventricle. This expression is also maintained during later pharyngula stages (P). (O') zath1 expression is lost from the cerebellar anlage in spg embryos (arrow) but expression recovers partially at later stages (arrow in P'). (Q) In wild-type embryos, expression of otx2 at pharyngula stages covers the midbrain and the MHB, in particular the concise stripe of the posterior cell row (arrow) marking the transition between the tectum and the cerebellar anlage. (Q') In spg embryos, expression of otx2 partially recovers within this particular posterior cell row (arrow) at late pharyngula stages. The spatial extent of the midbrain territory of otx2 is apparently smaller than in wild-type embryos. (R) Among the proneural genes, ngn1 is expressed in precursors of primary neurons in wild-type embryos at the beginning of somitogenesis. (R') ngn1 is strongly abrogated in mutant embryos. (S) sox17 is normally expressed within the endodermal precursors in a punctate pattern during gastrulation (inset: transversal section at 70% epiboly, showing pou2 expression restricted to the neuroectodermal layer). (S') sox17 expression is strongly affected in mutant embryos. (T) myod is expressed within the paraxial mesoderm and muscle precursors within somites during somitogenesis. (T') myod expression is strongly reduced in the somitic mesoderm of spg embryos but the paraxial domain seems unaffected.
