ECTODERMAL PATTERNING IN VERTEBRATE EMBRYOS

Recent molecular insights on how the ectodermal layer is patterned in vertebrates are reviewed. Studies on the induction of the central nerv ous system (CNS) by Spemann's Organizer led to the isolation of noggin and chordin. These secretory proteins function by binding to, and inh ibiting, ventral BMPs, in particular BMP-4. Neural induction can be co nsidered as the dorsalization of ectoderm, in which low levels of BMP- signaling result in CNS formation. At high levels of BMP signaling the ectoderm adopts a ventral fate and skin is formed. In Xenopus the for ming neural plate already has extensive dorsal-ventral (D-V) patternin g, and neural induction and D-V patterning may share common molecular mechanisms. At later stages sonic hedgehog (shh) plays a principal rol e in D-V patterning, particularly in the neural tube of the amniote em bryo. A great many transcription factor markers are available and mous e knockouts provide evidence of their involvement in the regional spec ification of the neural tube. Recent evidence indicating that differen tiation of posterior CNS is promoted by FGF, Wnt-3a, and retinoic acid is reviewed from the point of view of the classical experiments of Ni euwkoop that defined an activation and a transformation step during ne ural induction. (C) 1997 Academic Press.