2002).Īccording to the S&Z model, ACE occurs at the dorsal marginal zone in a quite early step of gastrulation. Moreover, the cell tracing by transplantation of the GFP-labeled dorsal marginal zone at ACE into a non-labeled ACE embryo revealed that the sliding between the mesoderm and the neuroectoderm proposed in the conventional model did not occur, supporting the S&Z gastrulation model (Koide et al. Consistent with this model, the body axis was formed from the dorsal marginal zone to the vegetal side of the embryo when the Xenopus eggs were embedded into gelatin to prevent the free rotation due to the shift of the center of gravity (Yanagi et al. After ACE, the head region is fixed at the dorsal equatorial region, and the notochord is formed posteriorly through the vegetal pole (Fig. In the case of Xenopus laevis, ACE occurs in the first 3 h of the whole 20-h process of gastrulation at 13 ☌. 1d), at which the whole region of neural tissues is determined. The developmental stage when the physical contact between the anterior-most neuroectoderm and the head organizer occurs is called “anterior contact establishment (ACE)” (Fig. Subsequently, the prospective neuroectoderm and the organizer become closer to establish physical contact with each other by subduction and zippering (S&Z) movement, which is probably performed by the combination of both epiboly (Keller 1978) and vegetal rotation (Winklbauer and Schürfeld 1999) (Fig. Eventually, the organizer tissue is located at the dorsal portion of the blastocoel floor by the onset of gastrulation (Fig. Both tissues move downward to the equatorial region by epiboly movement (Keller 1978). Briefly, the organizer is originally located in the blastocoel roof of the blastula adjacent to the prospective neuroectoderm around the animal pole (Fig. 2015) which is totally different from the conventional model drawn in a wide variety of both scientific papers and textbooks. Previously, we proposed an amphibian gastrulation model (Fig. Thus, it has been considered so far that the direction of notochord formation is opposite between amphibians and avians, and therefore, it is difficult to discuss the evolution of the morphogenetic movement. However, whereas the amphibian dorsal blastopore lip is thought to form a notochord anteriorly by animalward migration on the inner surface of the blastocoel roof, the chick Hensen’s node migrates and forms a notochord posteriorly. Therefore, these tissues are considered the equivalent structure, termed the organizer. 1997), and both tissues show a potent secondary axis-inducing ability upon transplantation (Spemann and Mangold 1924 Leikola, 1976). 1993 Smith and Harland 1992 Connolly et al. For example, the amphibian dorsal blastopore lip and the chick Hensen’s node express the same genes, such as goosecoid and noggin (Cho et al. While the molecular mechanisms involved in gastrulation are thought to be conserved among vertebrates, the morphogenetic movement is described as divergent. Gastrulation is a critical developmental event for producing three germ layers, three body axes, and the central nervous system in the embryo. Finally, by comparing amphibian gastrulation to gastrulation of protochordates and amniotes, we discuss the gastrulation movement evolutionarily conserved among chordates. 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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