Much attention has been focused on identifying the signals that specify the anterior nervous system in vertebrates, but surprisingly little is known about the factors that regulate the development of the spinal cord. The favoured model states that early signals specify anterior neural structures, then the more posterior structures develop in response to a transforming signal from the organizer region. Initially, fibroblast growth factor (FGF) signalling looked like a good candidate for the posteriorizing signal, because FGF was shown to activate posterior markers in Xenopus neural plate explants. However, other observations were difficult to reconcile with this idea; for example, blocking of FGF signalling in Xenopus had little effect on anteroposterior patterning. Now, in a paper published in Nature Cell Biology, Mathis et al. propose a different but equally important function for FGF signalling in spinal cord development.

The authors showed that in the chick, spinal cord progenitor cells reside in a region of the epiblast adjacent to the organizer, or Hensen's node. During development, the progeny of these cells normally become dispersed along the entire length of the spinal cord as the node progresses caudally. However, if FGF signalling is blocked in a subset of the progenitor cells using a dominant-negative FGF receptor (dnFGFr), the cells expressing this receptor fail to extend to the caudal end of the neural tube, perhaps indicating that they exit the node prematurely. This seems to be a cell-autonomous effect, because cells in the same embryo that express only the wild-type receptor still disperse normally to the tip of the tail bud. Mathis et al. suggest that FGF signalling maintains a stem zone of spinal-cord progenitors in the region surrounding Hensen's node. According to their model, the cells divide symmetrically, and half of the cells produced in each round of division are expelled into the neural plate. This seems to be a stochastic event, with the more rostrally positioned cells exiting the node first.

This study serves as a reminder of the importance of the precise coordination of cell movement and growth during development. Although the putative 'posteriorizing signal' still remains elusive, we have certainly gained valuable new insights into the role of FGF signalling in spinal cord development.