Cytoplasmic and molecular reconstruction of Xenopus embryos: synergy of dorsalizing and endo-mesodermalizing determinants drives early axial patterning.

Keiichi Katsumoto,Tomohiro Arikawa,Jun-Ya Doi,Hidefumi Fujii,Masao Sakai,Shin-Ichiro Nishimatsu

doi:10.1242/dev.01015

Abstract

Ablation of vegetal cytoplasm from newly fertilized Xenopus eggs results in the development of permanent blastula-type embryos (PBEs). PBEs cleave normally and develop into a very simple tissue consisting only of atypical epidermis. We tried to restore complete embryonic development in PBEs by cytoplasmic transplantation or by mRNA injection. We show a two-step reconstruction of the body plan. In the first step, PBEs injected with either marginal cytoplasm or synthetic VegT RNA restored gastrulation and mesoderm formation, but not axial patterning. Injection of Xwnt8 mRNA (acting upstream of beta-catenin and thus substitutes for the dorsal determinant) did not restore axial development in PBEs. Simultaneous injections of Xwnt8 and VegT into PBEs resulted in dorsal axis development, showing the synergy of these molecules in axial development. These results suggest that the mixing of two cytoplasmic determinants, i.e. the dorsal determinant in the vegetal pole and the endo-mesodermal determinant in the whole vegetal half, triggers the early axial developmental process in Xenopus embryos.

Highlights

Cytoplasmic determinants, which are located in particular regions of the egg, drive the cell fate during development
When more than 60% of vegetal cytoplasm and vegetal egg surface was deleted, the resulting embryos did not gastrulate but formed simple bag-like embryos (Fig. 1A; Table 1) called permanent blastula-type embryos (PBEs)
With PBEs we have an ideal recipient system for cytoplasm/ mRNA transplantation: PBEs do not have VegT and form only epidermal tissue, so it is relatively easy to detect the effect of transplantation sensitively

Summary

Introduction

Cytoplasmic determinants, which are located in particular regions of the egg, drive the cell fate during development. Cytoplasmic transplantation from a dorsal-vegetal blastomere at 16-cell stage Xenopus embryos into ventro-vegetal blastomere revealed the presence of a dorsal determinant(s) at this stage (Yuge et al, 1990). We have shown that deletion of 20-40% vegetal cytoplasm always results in gastrulating non-axial embryos (GNEs), which do not form any dorsal/neural structures (Sakai, 1996; Fujii et al, 2002). This axis deficiency can be fully restored by the transfer of the vegetal cytoplasm, providing a strong proof for dorsal determinants in the vegetal pole (Sakai, 1996)

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