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 have revealed that the endodermal fate becomes gradually fixed during the late blastula stages. Sox17 alpha, Mix. 1, Mixer and GATA-4 encode vegetal zygotic transcription factors with endoderm-inducing activity. The accumulation of their transcripts during the late blastula stages may cause determination of the endodermal fate. VegT, a T-box transcription factor, the maternal transcripts of which are vegetally localised, is also required for endoderm 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 around the midblastula transition (MBT) by maternal determinants. In phase 2, TGF beta signalling, possibly involving Xnr1, Xnr2 and Derriere, led to the activation of Mixer and GATA-4 in late blastula stages and to the reinforcement of the expression of Sox17 alpha and Mix.1. Overexpression of VegT in animal caps triggered a developmental programme qualitatively similar to that observed in vegetal blastomeres, except that Xnr1 and GATA-4 were not activated by the early gastrula stage. Conclusions: Our results support a two-step model for endoderm determination between fertilisation and the onset of gastrulation. The initial cell-autonomous activation of early endodermal genes by maternal determinants including, but not limited to, VegT is relayed by the action of zygotic TGF beta s such as Xnr1, Xnr2 and Derriere.