Downstream of the Organizer: The Xenopus Cement Gland as a Model for Organ Positioning and Differentiation
The correct positioning of organs during embryonic development requires multiple cues. The Xenopus cement gland, which forms where embryonic ectoderm and endoderm directly touch at the anterior of the embryo, is a simple model for organogenesis, allowing detailed analysis of this complex process. One way of thinking about how the cement gland is positioned is to define tissues and corresponding factors that alter cement gland formation. Several tissues including dorsal mesendoderm (Organizer), head endoderm and anterior neural plate induce cement gland, while others, such as ventral mesendoderm and chordamesoderm, inhibit its formation. One factor implicated in cement gland induction is BMP4, and it has been suggested that the cement gland forms at an intermediate level in a BMP4 gradient. An alternative view, presented here, is that the cement gland forms where large positional domains overlap. We suggest that these domains correspond to anterodorsal identity (AD), ventrolateral identity (VL), and ectodermal outer layer identity (EO), such that AD+VL+EO=CG. This “Venn diagram model” suggests that positional cues are integrated over time and space to activate cement gland differentiation genes. Genes corresponding to these domains are discussed.
KeywordsXenopus Laevis Xenopus Embryo Neural Induction Cement Gland Neural Fate
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