Mesoderm patterning and somite formation during node regression: differential effects of chordin and noggin

In Xenopus, one of the properties defining Spemann's organizer is its abili ty to dorsalise the mesoderm. When placed ajacent to prospective lateral/ve ntral mesoderm (blood, mesenchyme), the organizer causes these cells to ado pt a more axial/dorsal fate (muscle). It seems likely that a similar proper ty patterns the primitive streak of higher vertebrate embryos, but this has not yet been demonstrated clearly. Using quail/chick chimaeras and a panel of molecular markers, we show that Hensen's node (the amniote organizer) c an induce posterior primitive streak (prospective lateral plate) to form so mites (but not notochord) at the early neurula stage. We tested two BMP ant agonists, noggin and chordin (both of which are expressed in the organizer) , for their ability to generate somites and intermediate mesoderm from post erior streak, and find that noggin, but not chordin, can do this. Conversel y, earlier in development, chordin can induce an ectopic primitive streak m uch more effectively than noggin, while neither BMP antagonist can induce n eural tissue from extraembryonic epiblast. Neurulation is accompanied by re gression of the node, which brings the prospective somite territory into a region expressing BMP-2, -4 and -7. One function of noggin at this stage ma y be to protect the prospective somite cells from the inhibitory action of BMPs. Our results suggest that the two BMP antagonists, noggin and chordin, may serve different functions during early stages of amniote development. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.