EFFECTS OF RETINOIC ACID ON THE ENDODERM IN XENOPUS EMBRYOS

When Xenopus embryos from mid-tailbud to early tadpole stages were exp osed to retinoic acid (RA), the gut developed with an uncoiled, straig ht intestinal tube, morphogenesis of the liver and stomach was affecte d and intestinal epithelial cells developed without a brush border and alkaline phosphatase activity. However, the temporal and spatial expr ession pattern of X1Hbox 8, the only homeobox gene expressed in the en doderm was unaffected. In lateral plate mesodermal cells the expressio n of a-smooth muscle (SM) actin was delayed. A similar syndrome has be en reported in a study of embryos lacking functional FGF receptors in which it was proposed that the uncoiled intestinal tube and the delaye d differentiation of the intestinal muscle cells are causally related. Our results support this proposition and further suggest that mesench ymal-epithelial interactions concerned with regional specification of the endoderm may be impaired resulting in other defects in the gut.