Expresson of the Endodermal Marker, EcSox17, During Embryogenesis of the Direct Developing Frog, Eleutherodactylus coqui

In the model amphibian, Xenopus laevis, the HMG box transcription factor, Sox17, is expressed throughout the vegetal region that forms the endoderm. It is both necessary and sufficient for the endoderm formation. In order to study endoderm formation in the direct developing frog, Eleutherodactylus coqui, its Sox17 orthologue EcSox17 was cloned by Sean Willamson. To look at the expression of EcSox17, I designed primers and did RT-PCR on whole embryos. EcSox17 was expressed from cleavage to hatching of free-living froglet. EcSox17 RNA was present in the ovaries, an expression which has not been reported in any other vertebrate. Whole mount and bisected in situ hybridizations were done to look at the location of the EcSox17 RNA. EcSox17 was expressed at the blastoporal lip at all times and was not expressed in the yolk-rich vegetal core. RT-PCR on dissected pieces of early gastrulae however, indicated that the vegetal core expressed EcSox17, suggesting its contribution to the developing endoderm. I conclude that EcSox17 expression is ovarian as well as zygotic throughout the embryonic development. The transcript location at the blastoporal lip and perhaps throughout the vegetal region in early gastrula suggests its contribution to the definitive endoderm. Further experimentation is required to assess the function and necessity of EcSox17 in endoderm formation

Role of Thyroid Hormone in the Intestinal Development of Eleutherodactylus coqui

Thyroid hormone (TH) is required for the metamorphosis of the long, coiled tadpole gut in Xenopus laevis into a shorter mature adult gut. Eleutherodactylus coqui, a direct developing frog, lacks a tadpole. Its embryonic gut is a miniature adult form with a mass of yolky endodermal cells attached to the small intestine to provide nutrition. The requirement of TH for the gut development in E. coqui was tested in this study. Inhibition of TH synthesis with methimazole arrested gut development in its embryonic form. T3, the active form of TH, induced gut development. Embryos treated with methimazole failed to utilize the yolk in their nutritional endoderm, and survived for weeks without any further development. Acidification of the yolk platelet is an initial step in the breakdown of yolk in X. laevis. E. coqui embryos in methimazole failed to acidify their yolk platelets, but acidification was stimulated by TH indicating its role in yolk utilization. In X. laevis, TRβ is upregulated in response to TH and induces differentiation of the adult gut. Similarly, EcTRβ, the E. coqui orthologue, was upregulated by TH in the gut. EcTRβ expression was high in the nutritional endodermal cells indicating a direct role for TH in yolk utilization. The low expression level of EcSox17, an endodermal transcription factor, in these nutritional endodermal cells was consistent with the fact that these cells did not contribute to the definitive gut. This study indicated a novel role for TH in yolk utilization in addition to its conserved role in gut development and differentiation