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

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 β-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.

Cell‐autonomous and inductive processes among three embryonic domains control dorsal‐ventral and anterior‐posterior development of Xenopus laevis

“…1C), Xbra is expressed not in the VegT containing cytoplasm (which can be recognized as a large‐yolk containing sphere in the recipient PBE), but in the margin of the GNE‐derived cytoplasm. Injection of synthetic VegT mRNA into PBEs causes blastopore formation and Xbra expression (Katsumoto et al . 2004).…”

“…When VegT mRNA is coinjected with Xwnt8 mRNA into the same cell of PBEs, the resulting embryos develop dorsal structures and express the organizer gene chordin (Katsumoto et al . 2004).…”

“…1997); however, the vegetal cytoplasm most probably contains VegT mRNA as well as the DD. Injection of vegetal pole cytoplasm (VPC) into PBEs results in hyperdorsalized embryos, while in situ hybridization revealed that VPC injected PBEs contained VegT mRNA (Katsumoto et al . 2004).…”

“…The embryo forms a near normal dorsal axis. The vegetal pole cytoplasm contains both dorsal and endomesodermal determinants so that a single injection of the vegetal pole cytoplasm results in organizer formation, in a radial symmetrical manner (Katsumoto et al . 2004).…”

Gastrulation and pre-gastrulation morphogenesis, inductions, and gene expression: Similarities and dissimilarities between urodelean and anuran embryos

“…One is the three-signals model (Smith and Slack, 1983;Dale and Slack, 1987), in which localized distribution of maternal VegT (Zhang et al, 1998) and Wnt/ß-catenin in the oocyte cortex (Wylie et al, 1996;Moon and Kimelman, 1998) activate the GRN for meso-endoderm induction, and finally, signals emanating from dorso-vegetal cells induce and specify the DMZ. The other proposed model in Xenopus and Bufo is that the organizer is cell-autonomously specified by synergic action of maternal cytoplasmic determinants without inducing signals from dorso-vegetal cells (Sakai, 1996;Manes and Campos Casal, 2002;Fujii et al, 2002;Katsumoto et al, 2003;for review, see Sakai, 2008). Although these determinants are distributed more widely in the egg, a similar situation is also indicated in Cynops (Doi et al, 2000).…”