title: An anterior signalling centre in Xenopus revealed by the homeobox gene XHex.
abstract: BACKGROUND
  Signals from anterior endodermal cells that express the homeobox gene Hex initiate development of the most rostral tissues of the mouse embryo. The dorsal/anterior endoderm of the Xenopus gastrula, which expresses Hex and the putative head-inducing gene cerberus, is proposed to be equivalent to the mouse anterior endoderm. Here, we report the origin and signalling properties of this population of cells in the early Xenopus embryo.
  
  
  RESULTS
  Xenopus anterior endoderm was found to derive in part from cells at the centre of the blastocoel floor that express XHex, the Xenopus cognate of Hex. Like their counterparts in the mouse embryo, these Hex-expressing blastomeres moved to the dorsal side of the Xenopus embryo as gastrulation commenced, and populated deep endodermal adjacent to Spemann's organiser. Experiments involving the induction of secondary axes confirmed that XHex expression was associated with anterior development. Ventral misexpression of XHex induced ectopic cerberus expression and conferred anterior signalling properties to the endoderm. Unlike the effect of misexpressing cerberus, these signals could not neuralise overlying ectoderm.
  
  
  CONCLUSIONS
  XHex expression reveals the unexpected origin of an anterior signalling centre in Xenopus, which arises in part from the centre of the blastula and localises to the deep endoderm adjacent to Spemann's organiser. Signals originating from these endodermal cells impart an anterior identity to the overlying ectoderm, but are insufficient for neural induction. The anterior movement of Hex-expressing cells in both Xenopus and mouse embryos suggests that this process is a conserved feature of vertebrate development.
authors.affiliation: Division of Developmental Biology, National Institute for Medical Research, The Ridgeway, Mill Hill, NW7 1AA, UK. jonesm@icr.ac.uk.
authors.forename: C M
authors.initials: CM
authors.lastname: Jones
authors.forename: J
authors.initials: J
authors.lastname: Broadbent
authors.forename: P Q
authors.initials: PQ
authors.lastname: Thomas
authors.forename: J C
authors.initials: JC
authors.lastname: Smith
authors.forename: R S
authors.initials: RS
authors.lastname: Beddington
mesh_terms: D000818:Animals; D001755:Blastocyst / Q000166:cytology / Q000502:physiology*; D001757:Blastomeres / Q000166:cytology; D004627:Embryonic Induction / Q000502:physiology; D004707:Endoderm / Q000166:cytology; D005775:Gastrula / Q000502:physiology / Q000648:ultrastructure; D005801:Genes, Homeobox; D006257:Head / Q000196:embryology; D018398:Homeodomain Proteins / Q000235:genetics / Q000502:physiology*; D036341:Intercellular Signaling Peptides and Proteins; D051379:Mice; D008845:Microinjections; D009024:Morphogenesis / Q000235:genetics; D009928:Organ Specificity; D011506:Proteins / Q000235:genetics / Q000502:physiology; D012333:RNA, Messenger / Q000008:administration & dosage / Q000235:genetics; D013045:Species Specificity; D014157:Transcription Factors; D029867:Xenopus Proteins; D014982:Xenopus laevis / Q000196:embryology* / Q000235:genetics
chemical_list: C513080:Hhex protein, mouse; D018398:Homeodomain Proteins; D036341:Intercellular Signaling Peptides and Proteins; D011506:Proteins; D012333:RNA, Messenger; D014157:Transcription Factors; D029867:Xenopus Proteins; C100866:cer1 protein, Xenopus; C513078:hhex protein, Xenopus