(A) The organizer and the prospective neuroectoderm are aligned in tandem in the dorsal blastocoel roof at the blastula stage. The grey broken line indicates the blastocoel floor (equatorial) level of the embryo. The organizer and the prospective neuroectoderm are represented by red and blue, respectively, and the future A-P axis is indicated by numbers. Model of “subduction and zippering” movement. Curved arrows between A and P indicate the direction of axial structure formation. (B-7) An axial structure is progressively formed toward the posterior during the rest of the gastrulation movement. (B-6) The leading edge tissue migrates animally beyond the region of anterior contact. ACE occurs at equatorial region of very early gastrula. (B-4, 5) The zippering movement leads to physical contact between the blastocoel floor and the blastocoel roof. (B-2,3) The embryos undergo subduction movement. (A-6) Anterior contact establishment (ACE) occurs around the animal pole area of mid- to late-gastrula embryo. (A-2, 3, 4, 5) The organizer which is derived from the dorsal marginal zone, invaginates, involutes into the body, and then migrates toward the animal pole on the inner surface of the blastocoel roof. The organizer and the prospective neuroectoderm are represented by red and blue, respectively. (B) Proposed model of amphibian gastrulation “S&Z movement”. (A) Conventional mode of amphibian gastrulation. © 2015 The Authors Development, Growth & Differentiation published by Wiley Publishing Asia Pty Ltd on behalf of Japanese Society of Developmental Biologists.Ĭonventional model and Proposed model of amphibian gastrulation. Spemann organizer amphibian chordate gastrulation movement. The model also implies the possibility of constructing a common model of gastrulation among chordate species. After the contact is established, the dorsal axis is formed posteriorly, but not anteriorly. The contact is completed at the equator of early gastrulae and it continues throughout the gastrulation. Zippering movement, with forming Brachet's cleft, gradually closes the gap to establish the contact between them. Subduction makes a trench between the anterior organizer and the prospective neuroectoderm, and the tissues face each other via the trench. During the early step of gastrulation, the anterior organizer moves to establish the physical contact with the prospective neuroectoderm through the "subduction and zippering" movements. The organizer located at the blastocoel floor contributes to the anterior axial mesoderm including the prechordal plate, and the organizer at the dorsal lip ends up as the posterior axial mesoderm. In the model, the organizer is present at the blastocoel roof of blastulae, moves vegetally to locate at the region that lies from the blastocoel floor to the dorsal lip at the onset of gastrulation. Here we propose a unified model of amphibian gastrulation movement. However, we found that this physical contact was already established at the equatorial region of very early gastrula in a wide variety of amphibian species. The organizer is believed to involute inward and migrate animally to make physical contact with the prospective head neuroectoderm at the blastocoel roof of mid- to late-gastrula. The dorsal blastopore lip (known as the Spemann organizer) is important for making the body plan in amphibian gastrulation.
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