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The appearance of bottle cells at the dorsal vegetal/marginal boundary of Xenopus embryos marks the onset of blastopore formation. The conditions leading to this epithelial activity were investigated by inducing bottle cells ectopically in the animal region with VegT or different members of the transforming growth factor (TGF)-beta family. Morphological studies on the ectopic bottle cells indicate their close similarity to the endogenous bottle cells at the dorsal blastopore lip. The subepithelial cells of the induced animal region express mesodermal genes in a pattern reminiscent to that observed on the dorsal lip. Relating this expression pattern to the position of the ectopic bottle cells leads to the conclusion that bottle cells form in regions of high TGF-beta signalling. The specific inhibitory effects of cerberus on ectopically induced bottle cells revealed that nodal related growth factors are the intrinsic signals that elicit bottle cell formation in the normal embryo. In addition, fibroblast growth factor signalling is an essential precondition for this epithelial response as it is for mesoderm formation. We conclude that bottle cell formation in the epithelial layer of the gastrula is closely linked to mesodermal patterning in the subepithelial tissues.
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11025213
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Fig. 5. (A) Patterns of gene expression in normal embryos (a,b, vegetal view) and in embryos injected with the indicated amounts of mRNAs coding for activin (c,d, animal view) or Xnr1 (e,f, animal view). In normal gastrula embryos the dorsal marker goosecoid (gsc) is expressed directly on the lip (a). The transcripts of the panmesodermal marker Xbra form a ring in the marginal zone with a gap between the bottle cells and the Xbra positive region (b). Injection of activin or Xnr1 mRNA generates similar ectopic expression patterns of these markers in the animal cap: gsc is expressed near the ectopic bottle cells (c,e), whereas Xbra is induced at some distance to them (d,f). At an intermediate level of injected activin mRNA the pattern at the ectopic ‘lip’ closely resembles the wild type situation (compare b′ with d′). The schematic drawing illustrates the different domains abutting endogenous and ectopic bottle cells. Domain I represents the gap between the Xbra domain (domain II) and the bottle cell region whereas domain III represents a distalXbra negative domain. Arrows indicate the endogenous bottle cells in a,b and ectopic bottle cells in d. (B) Top: double in situ analysis of gsc (blue) and Xbra (magenta) in control (a) and in embryos injected with activin mRNA (b). Whole-mount in situ hybridization was performed on dissected embryos to visualize internal patterns of transcript distribution. Bottom: interpretative drawings of the expression domains in (a) and (b). On the dorsal side of uninjected embryos goosecoid signal is found in deeper mesendodermal regions whereas Xbra-transcripts occur in more peripheral regions (a). (b) Ectopic internal patterns in an embryo injected with activin mRNA. The embryo is cut perpendicular to the dorsal–ventral axis. As in the control Xbra and gsc transcripts are separated from each other (arrow). Gsc is expressed in the tissue underneath the bottle cell field but cannot be found in the bottle cells themselves. Abbreviations: An, animal; bc, blastocoel; D, dorsal; ect, ectopic; end, endogenous; ep, epithelium; Le, left; Ri, right; V, ventral; Veg, vegetal. Asterisks indicate the injection sites in (A,B).
