Collectively, these results are consistent with the S&Z gastrulation model and identify the embryonic region sufficient for construction of the complete dorsal structure. Furthermore, a blastocoel roof explant of the blastula, which should contain the organizer and the prospective neuroectoderm in the S&Z model, autonomously underwent gastrulation and formed the complete dorsal structure. To investigate this possibility, we conducted stepwise tissue deletions using Xenopus laevis embryos and revealed that the dorsal one-third of the marginal zone had the ability to form the complete dorsal structure by itself. According to this model, the body axis is derived from limited regions of the dorsal marginal zone at ACE. The developmental stage when contact between the head organizer and the anterior-most neuroectoderm is established is called “anterior contact establishment (ACE).” After ACE, the A-P body axis elongates posteriorly. In this model, the organizer and the prospective neuroectoderm are originally localized in the blastula’s blastocoel roof, and these embryonic regions move downward to make physical contact of their inner surfaces with each other at the dorsal marginal zone. Previously, we proposed a novel amphibian gastrulation model, the “subduction and zippering (S&Z) model”. However, the morphological movement during gastrulation appears to be divergent across species, making it difficult to discuss the evolution of the process. Gastrulation is a critical event whose molecular mechanisms are thought to be conserved among vertebrates.
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