A two-step model for the fate determination of presumptive endodermal blastomeres in Xenopus embryos

Background: In Xenopus, the endoderm germ layer is derived from the vegetal blastomeres of cleavage-stage embryos, Cell transplantation experiments ha ve revealed that the endodermal fate becomes gradually fixed during the lat e blastula stages. Sox17 alpha, Mix. 1, Mixer and GATA-4 encode vegetal zyg otic transcription factors with endoderm-inducing activity. The accumulatio n of their transcripts during the late blastula stages may cause determinat ion of the endodermal fate. VegT, a T-box transcription factor, the materna l transcripts of which are vegetally localised, is also required for endode rm formation. Results: We analysed the events leading to the progressive accumulation of the transcripts for Sox17 alpha, Mix. 1, Mixer and GATA-4. Two phases could be distinguished in the endodermal programme. In phase 1, Sox17 alpha, Mix . 1, and the genes encoding transforming growth factor P-related signalling molecules Xnr1, Xnr2 and Derriere were activated cell-autonomously at arou nd the midblastula transition (MBT) by maternal determinants. In phase 2, T GF beta signalling, possibly involving Xnr1, Xnr2 and Derriere, led to the activation of Mixer and GATA-4 in late blastula stages and to the reinforce ment of the expression of Sox17 alpha and Mix.1. Overexpression of VegT in animal caps triggered a developmental programme qualitatively similar to th at observed in vegetal blastomeres, except that Xnr1 and GATA-4 were not ac tivated by the early gastrula stage. Conclusions: Our results support a two-step model for endoderm determinatio n between fertilisation and the onset of gastrulation. The initial cell-aut onomous activation of early endodermal genes by maternal determinants inclu ding, but not limited to, VegT is relayed by the action of zygotic TGF beta s such as Xnr1, Xnr2 and Derriere.