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The endoderm gives rise the respiratory and digestive tract epithelia as well as associated organs such as the liver, lungs and pancreas. Investigations examining the molecular basis of embryonic endodermal patterning and organogenesis have been hampered by the lack of regionally expressed molecular markers in the early endoderm. By differentially screening an arrayed cDNA library, combined with an in situ hybridization screen we identified 13 new genes regionally expressed in the early tailbudendoderm of the Xenopus embryo. The putative proteins encoded by these cDNAs include a cell surface transporter, secreted proteins, a protease, a protease inhibitor, an RNA-binding protein, a phosphatase inhibitor and several enzymes. We find that the expression of these genes falls into one of three re-occurring domains in the tailbudembryo; (1). a ventralmidgut, (2). posterior to the midgut and (3). in the dorsal endoderm beneath the notochord. Several of these genes are also regionally expressed at gastrula and neurula stages and appear to mark territories that were previously only predicted by the endoderm fate map. This indicates that there is significant positional identity in the early endoderm long before stages 28-32 when regional specification of the endoderm is thought to occur. These new genes provide valuable tools for studying endodermal patterning and organogenesis in Xenopus.
Fig. 2. Genes expressed in the Xenopus ventralmidgut from stages 22–39. In situ hybridizations with antisense Vito [slc5a8.1], β3 ATPase subunit [atp1b3], FigoB [nickipsd], Sam68 [khdrbs1], Pil [ppp1r1a] and EphrinB1 [efnb1] probes. Black arrows indicate expression in the liver diverticulum and the red arrows indicate the presumptive gall bladder. The thin black line indicates the position along the anterior-posterior axis where the cross sections on the right were taken. For the whole mount preparations anterior is to the right and in sections dorsal is up. The spot of dark staining with Vito, and Figo on the flank of the tadpole embryos is the developing pronephros.
Fig. 3. Genes expressed in the Xenopus posteriorendoderm from stages 22–39. Whole mount in situ hybridizations with antisense IMP2 [impdh1], Chito [ctbs], Endocut [darmin] and complement C3 [c3] probes. The thin black line indicates the position along the anterior-posterior axis where the cross sections on the right were taken. For the whole mount preparations anterior is to the right and in sections dorsal is up.
Fig. 4. Genes expressed in Xenopus the dorsal endoderm from stages 22–39. In situ hybridization with antisense Fetuinish [fetub], NDRG1 [ndrg1] and Vent-2 [ventx2.2] probes. The thin black line indicates the position along the anterior-posterior axis where the cross sections on the right were taken. For the whole mount preparations anterior is to the right and in sections dorsal is up. In NDRG1 hybridized embryos, the dark staining on the external flank of the embryo is the developing kidney.
Fig. 5. Gastrula and neurula stage expression. In situ hybridization to gastrula and neurual stage Xenopus embryos with Endocut [darmin], Vito [slc5a8.1] , β3 [] and Fetuinish (fetub) antisense probes. The two left panels show whole mount in situ preparations to stage 11 gastrula embryos, from a vegetal view (extreme left) and a mid-sagital section, with dorsal/anterior to the left. The superficialendoderm of the blastopore lip (red arrow head) expresses Vito and β3 but not Endocut. The stage 15–18 embryos are bisected mid-sagitally and shown with anterior to the left and dorsal up. The stage 20 embryos are shown in cross section with dorsal up.
Fig. 6. Gene expression in organ buds of the stage 42 gut tube. The top panel show a schematic of an isolated stage 42 Xenopus gut tube indicating the lung buds (lu; blue), Liver bud (li; red), gall bladder bud (gb), pancreatic bud (p; gene) and intestine (in). The lower eight panels show whole mount in situ preparations of isolated stage 42 gut tubes with NDRG1 [ndrg1], Sam68 [khdrbs1], Fetuinish [fetub] , Vent-2 [ventx2.2], IMP2 [impdh1], Chito [ctbs] , Endocut [darmin] and C3 [c3] antisense probes.
