THE PRECHORDAL REGION LACKS NEURAL INDUCING ABILITY, BUT CAN CONFER ANTERIOR CHARACTER TO MORE POSTERIOR NEUROEPITHELIUM

The avian equivalent of Spemann's organizer, Hensen's node, begins to lose its ability to induce a nervous system from area opaca epiblast c ells at stage 4(+), immediately after the full primitive streak stage, From this stage, the node is no longer able to induce regions of the nervous system anterior to the hindbrain, Stage 4(+) is marked by the emergence from the node of a group of cells, the prechordal mesendoder m. Here we have investigated whether the prechordal region possesses t he lost functions of the organizer, using quail-chick chimaeras to dis tinguish graft- and host-derived cells, together with several region-s pecific molecular markers. We find that the prechordal region does not have neural inducing ability, as it is unable to divert extraembryoni c epiblast cells to a neural fate, However, it can confer more anterio r character to prospective hindbrain cells of the host, making them ac quire expression of the forebrain markers tailless and Otx-2. It can a lso rescue the expression of Krox-20 and Otx-2 from nervous system ind uced by an older (stage 5) node in extraembryonic epiblast, We show th at these properties reflect a true change of fate of cells rather than recruitment from other regions, The competence of neuroectoderm to re spond to anteriorizing signals declines by stages 7-9, but both poster iorizing signals and the ability of neuroectoderm to respond to them p ersist after this stage.