Dissecting planar and vertical organiser signals in early chick neural development.

Early neural development involves a combination of planar signals from the vertebrate organiser and vertical signals from its derived structures, the prechordal plate and notochord, together with early permissive signals from the hypoblast and developing mesendoderm. However, the relative contribution of each organiser-derived structure to neural development is not clear. Here, we isolate anterior tissues from the chick primitive streak at successively later stages of development, to dissect the temporal order of signals from the organiser and its derivatives. Our results show that acquisition of neural identity occurs gradually and that exposure to planar signals from the developing node is necessary for neural plate specification. We also show that planar node-derived signals are required for AP patterning in isolated anterior tissues and give evidence that early neural tissue is of anterior character that subsequently becomes caudalised (in part) by signals from the developing node. However, anterior neural identity is lost without long-term contact with vertical signals from the axial mesendoderm. These results reveal a progressive refinement of neural identity and patterning through a combination of planar and vertical signalling.