The specification and growth factor inducibility of the pronephric glomus in Xenopus laevis

We report a study on the specification of the glomus, the filtration device of the amphibian pronephric kidney, using an explant culturing strategy in Xenopus laevis. Explants of presumptive pronephric mesoderm were dissected from embryos of mid-gastrula to swimming tadpole stages. These explants were cultured within ectodermal wraps and analysed by RT-PCR for the presence of the Wilm's Tumour-1 gene, xWT1, a marker specific for the glomus at the stages analysed, together with other mesodermal markers. We show that the glomus is specified at stage 12.5, the same stage at which pronephric tubules are specified. We have previously shown that pronephric duct is specified somewhat later, at stage 14. Furthermore, we have analysed the growth factor inducibility of the glomus in the presence or absence of retinoic acid (RA) by RT-PCR. We define for the first time the conditions under which these growth factors induce glomus tissue in animal cap tissue. Activin together with high concentrations of RA can induce glomus tissue from animal cap ectoderm. Unlike the pronephric tubules, the glomus can also be induced by FGF and RA.