CHORDIN REGULATES PRIMITIVE STREAK DEVELOPMENT AND THE STABILITY OF INDUCED NEURAL CELLS, BUT IS NOT SUFFICIENT FOR NEURAL INDUCTION IN THECHICK-EMBRYO

We have investigated the role of Bone Morphogenetic Protein 4 (BMP-4) and a BMP antagonist, chordin, in primitive streak formation and neura l induction in amniote embryos. We show that both BMP-4 and chordin ar e expressed before primitive streak formation, and that BMP-4 expressi on is downregulated as the streak starts to form. When BMP-4 is misexp ressed in the posterior area pellucida, primitive streak formation is inhibited. Misexpression of BMP-4 also arrests further development of Hensen's node and axial structures. In contrast, misexpression of chor din in the anterior area pellucida generates an ectopic primitive stre ak that expresses mesoderm and organizer markers. We also provide evid ence that chordin is not sufficient to induce neural tissue in the chi ck. Misexpression of chordin in regions outside the future neural plat e does not induce the early neural markers L5, Sox-3 or Sox-2. Further more, neither BMP-4 nor BMP-7 interfere with neural induction when mis expressed in the presumptive neural plate before or after primitive st reak formation. However, chordin can stabilise the expression of early neural markers in cells that have already received neural inducing si gnals. These results suggest that the regulation of BMP signalling by chordin plays a role in primitive streak formation and that chordin is not sufficient to induce neural tissue.