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???displayArticle.abstract??? Bottle cell-driven blastopore lip formation externally marks the initiation of gastrulation in amphibian embryos. The blastopore groove is formed when bottle cells undergo apical constriction and transform from cuboidal to flask-shaped. Apical constriction is sufficient to cause invagination and is a highly conserved mechanism for sheet bending and folding during morphogenesis; therefore, studying apical constriction in Xenopus bottle cells could provide valuable insight into this fundamental shape change. Initially described over a century ago, the dramatic shape change that occurs in bottle cells has long captured the imaginations of embryologists. However, only recently have investigators begun to examine the cellular and molecular mechanisms underlying bottle cell apical constriction. Bottle cell apical constriction is driven by actomyosin contractility as well as by endocytosis of the apical membrane. The Nodal signaling pathway, Wnt5a, and Lgl1 are all required for bottle cell formation, but how they induce subcellular changes resulting in apical constriction remains to be elucidated. Xenopus bottle cells now represent an excellent vertebrate system for the dissection of how molecular inputs can drive cellular outputs, specifically the cell shape change of apical constriction.
Figure 1.
Bottle cell formation as the first external sign of Xenopus gastrulation. Top, vegetal view of blastopore formation, with bottle cells forming initially in the dorsal marginal zone (DMZ), then laterally and ventrally to form the circular blastopore. Arrows mark the extent of apically constricting bottle cells. Bottom, midsagittal confocal images of bottle cells immunostained with α-tubulin antibody. Embryos are oriented apical down and animal to the right. Arrows point to center of blastopore invagination. St., stage. (Reprinted with permission from Ref 3. Copyright 2007 Elsevier)
Figure 2.
Holtfreter's classic experiment of blastoporal cells incorporating, then invaginating, into an endodermal substrate. (Reprinted with permission from Ref 16. Copyright 1944 JD Wiley and Sons)
Figure 3.
F-actin and activated Myosin (pMLC) are enriched apically in bottle cells, whereas microtubules (α-tubulin) are arranged in apicobasal arrays. Midsagittal confocal images; embryos are oriented apical down and animal to the right. Scale bar = 50 µm. (Reprinted with permisson from Ref 3. Copyright 2007 Elsevier)
Figure 4.
Schematic of the cytoskeletal mechanisms underlying bottle cell apical constriction, as described in Refs 3 and 24. Actomyosin contractility is the main driving force, but endocytosis is also required for efficient constriction later in the process. Adherens junctions have not been directly implicated but are drawn in at their presumed subcellular location.
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