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. However, the relative contribution of each structure to neural development is not clear. Here, we isolate anterior tissues from the primitive streak at successively later stages of development, to identify the extent of patterning that can occur prior to, during, and after the formation of the organiser and its later 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 which subsequently becomes caudalised by signals (in part) 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 previously unappreciated level of autonomy in anterior neural development in the absence of node derived tissues. Summary statementCulture of isolated anterior tissues from the chick embryo reveal the roles of planar and vertical organiser signals for neural specification and anteroposterior patterning and maintenance.