Anteroposterior Patterning in Xenopus Embryos: Egg Fragment Assay System Reveals a Synergy of Dorsalizing and Posteriorizing Embryonic Domains

Abstract

Two distinct types of axis lacking embryos resulted from partial deletion of the vegetal part of early one-cell-stage embryos. When the deleted volume was 20-40% (relative surface area), the embryos underwent ventral-type gastrulation and formed ventral mesodermal tissues. When the deleted volume was more than 60%, the embryo did not gastrulate nor make mesodermal structures (M. Sakai, 1996, Development 122, 2207-2214). We have designated these two types of embryos as "gastrulating nonaxial embryos (GNEs)" and "permanent blastula-type embryos (PBEs)," respectively. Using these embryos as recipients, a series of Einsteck transplantation experiments were carried out to investigate mechanisms controlling anteroposterior patterning during early Xenopus development. GNEs receiving dorsal marginal zone (DMZ) transplants (GNE/DMZs) elongated and formed posteriorized phenotypes, which had muscle cells, melanocytes, and tail fins. In contrast, PBE/DMZs did not elongate but formed cement glands and brain-like structures showing strong anteriorization. Simultaneous transplantation of the cells from various regions of normal embryos with the DMZ into PBEs revealed that the entire vegetal half of normal embryos, except for the DMZ, showed posteriorizing activity. These results strongly suggest that anteroposterior patterning in Xenopus is not achieved solely by the dorsal marginal zone (the Spemann organizer), but instead by a synergistic mechanism of the dorsalizing domain (DMZ) and the posteriorizing domain (the entire vegetal half except for the DMZ).

Comparison of induction during development between Xenopus tropicalis and Xenopus laevis

“…There have been many developmental studies of morphogen gradients and axes patterning (Bourillot et al, 2002, Dubrulle et al, 2001, Dyson and Gurdon, 1998, Fujii et al, 2002, Gamse and Sive, 2000, Gamse and Sive, 2001, Green et al, 1997, Green et al, 1992, Gurdon et al, 1996, Gurdon et al, 1999, Ninomiya et al, 2004, Tabata and Takei, 2004. Morphogen gradients were thought to regulate the positioning of cells in a developing embryo, as well as controlling gene expression and tissue-specific differentiation.…”

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

“…1A (Fujii et al, 2002). LiCl treatment (0.3 M for 6 minutes at 32-cell stage) of PBEs did not induce chordin expression (0/17, data not shown).…”

“…As recipients for cytoplasmic transfer, we used PBEs, which were made by ablating more than 60% of the vegetal egg surface from an early one-cell stage embryo (Fujii et al, 2002). PBEs do not gastrulate, or express dorsal, neural and endomesodermal genes.…”

“…PBEs do not gastrulate, or express dorsal, neural and endomesodermal genes. They express the epidermal marker EpK, and develop into simple epidermal tissues (Fujii et al, 2002). Therefore, PBEs most probably lack two types of determinants: the dorsal determinants in the vegetal pole and the marginal determinants.…”

“…The dorsal determinant(s) is localized in the vegetal pole at the early onecell stage (Fujisue et al, 1993;Holowacz and Elinson, 1993), thereafter it disappears from the vegetal region and reappears in the dorsal region, without any dorsal inducing activity being detected in the ventral subequatorial region at any stage (Yuge et al, 1990;Fujisue et al, 1993). Furthermore, 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).…”