TGF-BETA SIGNALS AND A PREPATTERN IN XENOPUS-LAEVIS ENDODERMAL DEVELOPMENT

We have analyzed two gene products expressed in the early endoderm of Xenopus laevis: Xlhbox-8, a pancreas-specific transcription factor and intestinal fatty acid binding protein (IFABP), a marker of small inte stinal epithelium. Expression of the pancreas marker relies on cell si gnaling mediated by both the TGF-beta and FGF classes of secreted pept ide growth factors, whereas, expression of the more posterior small in testinal marker does not, Endodermal explants devoid of mesoderm expre ss both markers in a regionalized manner. Cortical rotation is require d for the expression of the more anterior marker, Xlhbox-8, but not fo r the small intestinal marker, IFABP. These findings suggest that endo dermal patterning is dependent, in part, on the same events and signal s known to play important roles in mesodermal development, Furthermore , inhibition of TGF-beta signaling in the endoderm leads to ectopic ex pression of both mesodermal and ectodermal markers, suggesting that TG F-beta signaling may play a general role in the segregation of the thr ee embryonic germ layers.