PREVENTING THE LOSS OF COMPETENCE FOR NEURAL INDUCTION - HGF SF, L5 AND SOX-2/

The response to neural induction depends on the presence of inducing s ignals and on the state of competence of the responding tissue, The ep iblast of the chick embryo loses its ability to respond to neural indu ction by the organizer (Hensen's node) between stages 4 and 4(+). We f ind that the pattern of expression of the L5(220) antigen closely mirr ors the changes in competence of the epiblast in time and in space, Fo r the first time, we describe an experiment that can extend the period of neural competence: when L5(220) expression is maintained beyond it s normal time by implanting HGF/SF secreting cells, the competence to respond to Hensen's node grafts is retained, The host epiblast forms a non-regionalized neural tube, which expresses the pan-neural marker S OX-2 (a Sry-related transcription factor) but not any region-specific markers for the forebrain, hindbrain or spinal cord. Although HGF/SF s ecreting cells can mimic signals from Hensen's node that maintain L5 e xpression, they cannot rescue the ability of the node to induce anteri or structures (which is normally lost after stage 4), The ectoderm may acquire stable neural characteristics during neural induction by goin g through a hierarchy of states: competence, neuralization and regiona lization, Our findings allow us to start to define these different sta tes at a molecular level, and show that the competence to respond to n eural induction is not entirely autonomous to the responding cells, bu t can be regulated by extracellular signalling molecules.