EARLY SPECIFICATION OF INTESTINAL EPITHELIUM IN THE CHICKEN-EMBRYO - A STUDY ON THE LOCALIZATION AND REGULATION OF CDXA EXPRESSION

CdxA, a chicken homeobox-containing gene related to caudal in Drosophi la, has been implicated in the regionalization of endoderm. It is repo rted here that, in the development of the chicken embryo, CdxA express ion appears in the endoderm at day 1.5 of development as bilateral ban ds on either side of the splanchnopleure which later contribute to int estinal epithelium. The CdxA-expressing area extends medially and caud ally as formation of the gut tube progresses. It is also shown that th e rostral limit of GdxA expression demarcates the boundary between sto mach and duodenum after day 3 of development. CdxA is not expressed in digestive tract appendages which open into the intestine, such as pan creas, liver and allantois. Early restriction of CdxA expression in in testinal lineage suggests that the intestinal specification involving CdxA expression commences before the gut tube is formed. The expressio n of CdxA in epithelial-mesenchymal tissue recombinants suggests that mesenchymal influence regulating CdxA expression plays an important ro le in confirming the boundary between the stomach and intestine. Chron ological change in the spatial distribution of CdxA transcripts and th e results of tissue recombination experiments, together with precise f ate maps of early endoderm and splanchnic mesoderm, lead to a model of mechanisms by which intestinal specification is brought about.