Cell interactions underlying notochord induction and formation in the chick embryo

The development of the notochord in the chick is traditionally associated w ith Hensen's node (the avian equivalent of the organizer). However, recent evidence has shown that two areas outside the node (called the inducer and responder) are capable of interacting after ablation of Hensen's node to fo rm a notochord. It was not clear from these studies what effect (if any) si gnals from these areas had on normal notochord formation. A third area, the postnodal region, may also contribute to notochord formation, although thi s has also been questioned. Using transection and grafting experiments, we have evaluated the timing and cellular interactions involved in notochord i nduction and formation in the chick embryo. Our results indicate that the r ostral primitive streak, including the node, is not required for formation of the notochord in rostral blastoderm isolates transected at stages 3a/b. In addition, neither the postnodal region nor the inducer is required for t he induction and formation of the most rostral notochordal cells. However, inclusion of the inducer results in considerable elongation of the notochor d in this experimental paradigm. Our results also demonstrate that the resp onder per se is not required for notochord formation, provided that at leas t the inducer and postnodal region are present, although in the absence of the responder, formation of the notochord occurs far less frequently. We al so show that the node is not specified to form notochord until stage 4 and concomitant with this, the inducer loses its ability to induce notochord fr om the responder. The coincident timing of these changes in the node and in ducer suggests that notochord specification and the activity of the inducer are regulated through a negative feedback loop. We propose a model relatin g our results to the induction of head and trunk organizer activity in the node. (C) 2001 Wiley-Liss, Inc.