| The kidney is derived from the intermediate mesoderm (IM), a strip of mesoderm that lies between the somites and the lateral plate. In this thesis, the chick pronephros was used as a model system to identify the critical time periods during which patterning of the IM takes place. In the third chapter, we used transplantation and explant culture to show that the avian IM is specified immediately after gastrulation. Transplantation experiments revealed that the lateral plate contains an activity that can repress IM formation in tissues that are already specified. In contrast, combining lateral plate and paraxial tissue in vivo or in vitro led to differentiation of IM in the paraxial, but not lateral plate mesoderm. Based on these results and those of others we proposed that a gradient of a secreted signal that is distributed from the lateral plate to the axial tissues, modulates trunk mesoderm differentiation. In chapter four, we provide evidence that the forkhead transcription factors Foxc1 and Foxc2, play a role in differentiation of paraxial versus IM fates. First, mice mutant for both Foxc1 and Foxc2 so not develop somites and show ectopic differentiation of IM in the paraxial domain. Conversely, we show that ectopic expression of Foxc1 or Foxc2 in chickens inhibits IM and promotes the formation of ectopic somites. Taken together, the data indicate that Foxc1 and Foxc2 regulate the establishment of paraxial mesoderm versus IM in the vertebrate embryo. In chapter 5, we have tested the hypothesis that Bmp (bone morphogenic protein) signals in the mesoderm are responsible for the gradient of activity that we characterized in chapter 3. Culture of somitic explants with exogenous Bmp-2 demonstrates that the mesoderm responds to manipulation of Bmp levels in a dose-dependent manner. At low doses, Bmp-2 promotes differentiation of IM, whereas at high doses it promotes differentiation of lateral plate. Furthermore, we have demonstrated that Bmp acts in a cell autonomous, but indirect manner. In light of this data, we postulate that the mechanism by which low doses of Bmp activate IM gene expression is by inhibition of a repressor present in paraxial mesoderm. |
