FGF SIGNALING IN THE EARLY SPECIFICATION OF MESODERM IN XENOPUS

We have examined the role of FGF signalling in the development of musc le and notochord and in the expression of early mesodermal markers in Xenopus embryos. Disruption of the FGF signalling pathway by expressio n of a dominant negative construct of the FGF receptor (XFD) generally results in gastrulation defects that are later evident in the formati on of the trunk and tail, though head structures are formed nearly nor mally. These defects are reflected in the loss of notochord and muscle . Even in embryos that show mild defects and gastrulate properly, musc le formation is impaired, suggesting that morphogenesis and tissue dif ferentiation each depend on FGF. The XFD protein inhibits the expressi on of the immediate early gene brachyury throughout the marginal zone, including the dorsal side; it does not, however, inhibit the dorsal l ip marker goosecoid, which is expressed in the first involuting mesode rm at the dorsal side that will underlie the head. The XFD protein als o inhibits Xpo expression, an immediate early marker of ventral and la teral mesoderm. These results suggest that FGF is involved in the earl iest events of most mesoderm induction that occur before gastrulation and that the early dorsal mesoderm is already composed of two cell pop ulations that differ in their requirements for FGF.