Development of chick axial mesoderm: specification of prechordal mesoderm by anterior endoderm-derived TGF beta family signalling

Two populations of axial mesoderm cells can be recognised in the chick embr yo, posterior notochord and anterior prechordal mesoderm, We have examined the cellular and molecular events that govern the specification of prechord al mesoderm, We report that notochord and prechordal mesoderm cells are int ermingled and share expression of many markers as they initially extend out of Hensen's node. In vitro culture studies, together with in vivo grafting experiments, reveal that early extending axial mesoderm cells are labile a nd that their character may be defined subsequently through signals that de rive from anterior endodermal tissues. Anterior endoderm elicits aspects of prechordal mesoderm identity in extending axial mesoderm by repressing not ochord characteristics, briefly maintaining gsc expression and inducing BMP 7 expression. Together these experiments suggest that, in vivo, signalling by anterior endoderm may determine the extent of prechordal mesoderm. The t ransforming growth factor beta (TGF beta) superfamily members BMP2 BMP3, BM P7 and activin, all of which are transiently expressed in anterior endoderm mimic distinct aspects of its patterning actions. Together our results sug gest that anterior endoderm-derived TGF beta s may specify prechordal mesod erm character in chick axial mesoderm.