NEURAL INDUCTION AND REGIONALIZATION IN THE CHICK-EMBRYO

Induction and regionalisation of the chick nervous system were investi gated by transplanting Hensen's node into the extra-embryonic region ( area opaca margin) of a host embryo. Chick/quail chimaeras were used t o determine the contributions of host and donor tissue to the supernum erary axis, and three molecular markers, Engrailed, neurofilaments (an tibody 3A10) and XIHbox1/Hox3.3 were used to aid the identification of particular regions of the ectopic axis. We find that the age of the n ode determines the regions of the nervous system that form: young node s (stages 2-4) induce both anterior and posterior nervous system, whil e older nodes (stages 5-6) have reduced inducing ability and generate only posterior nervous system. By varying the age of the host embryo, we show that the competence of the epiblast to respond to neural induc tion declines after stage 4. We conclude that during normal developmen t, the initial steps of neural induction take place before stage 4 and that anteroposterior regionalisation of the nervous system may be a l ater process, perhaps associated with the differentiating notochord. W e also speculate that the mechanisms responsible for induction of head CNS differ from those that generate the spinal cord: the trunk CNS co uld arise by homeogenetic induction by anterior CNS or by elongation o f neural primordia that are induced very early.