Temporal sequence of gene expression leading caudal prosencephalon to develop a midbrain/hindbrain phenotype

Transplantation of prosomeres 1-2 into the cerebellar plate were used, by using chick/quail chimeras, to analyse the temporal sequence of the genetic cascade leading the graft to develop a midbrain/hindbrain phenotype. Our results show that (1) at Hamburger and Hamilton (HH) stage 13, Pax2 and En2 are already induced within the graft, before all other genes of the cascade, whereas misexpression of Fgf8 is also observed within the contiguous host cerebellar plate; (2) within the graft, Otx2 repression and Gbx2 induction (see Hidalgo-Sánchez et al. [1999] Development 126:3191-3203) are secondary events that affect, from stages HH14-15, the areas in contact with the host Gbx2/Fgf8-expressing cerebellar plate; (3) at these stages, the repressed Otx2 territory extends beyond the areas induced to express Gbx2, with the two territories not abutting before HH17-18; (4) Fgf8 expression becomes progressively induced within the Otx2-repressed/Gbx2-induced territory, starting at HH15-16. Our results support the hypothesis that the host-Gbx2/graft-Otx2 interface could trigger the genetic cascade induced within the graft and that the Gbx2-induced domain could play a key role during the establishment of the induced intragraft midbrain/hindbrain boundary.