FIBROBLAST GROWTH-FACTOR IS A DIRECT NEURAL INDUCER, WHICH COMBINED WITH NOGGIN GENERATES ANTERIOR-POSTERIOR NEURAL PATTERN

Neural tissue in developing Xenopus embryos is induced by signals from the dorsal mesoderm, Induction of anterior neural tissue could be med iated by noggin, a secreted polypeptide found in dorsal mesoderm. We s how that bFGF, a known mesoderm inducer of blastula staged ectoderm, i nduces neural tissue from gastrula stage ectoderm, The type of neural tissue induced by bFGF from stage 10.25 ectoderm is posterior, as mark ed by Hox B9 expression. When bFGF and noggin are combined on early ga strula stage ectoderm, a more complete neural pattern is generated and no mesodermal tissue is detected, Explants treated with noggin and bF GF elongate and display distinct anterior and posterior ends marked by otx2 and Hox B9 expression, respectively, Furthermore, treatment of e arly gastrula ectoderm with noggin and bFGF results in the induction o f En-2, a marker of the midbrain-hindbrain junction and Krox 20, a mar ker of the third and fifth rhombomeres of the hindbrain, Neither of th ese genes is induced by noggin alone or bFGF alone at this stage, sugg esting a synergy in anterior-posterior neural patterning. The response of later gastrula (stage 11-12) ectoderm to bFGF changes so that Krox 20 and En-2 are induced by bFGF alone, while induction of more poster ior tissue marked by Hox B9 is eliminated, The dose of bFGF affects th e amount of neural tissue induced, but has little effect on the anteri or-posterior character, rather the age of the ectoderm treated is the determinant of the response. Thus, an FGF signal may account for poste rior neural induction, and anterior-posterior neural patterning could be partly explained by the actions of noggin and FGF, together with th e changing response of the ectoderm to these factors.