title: Initiation of gastrulation in the mouse embryo is preceded by an apparent shift in the orientation of the anterior-posterior axis.
abstract: BACKGROUND
  It is generally assumed that the migration of anterior visceral endoderm (AVE) cells from a distal to a proximal position at embryonic day (E)5.5 breaks the radial symmetry of the mouse embryo, marks anterior, and conditions the formation of the primitive streak on the opposite side at E6.5. Transverse sections of a gastrulating mouse embryo fit within the outline of an ellipse, with the primitive streak positioned at one end of its long axis. How the establishment of anterior-posterior (AP) polarity relates to the morphology of the postimplantation embryo is, however, unclear.
  
  
  RESULTS
  Transverse sections of prestreak E6.0 embryos also reveal an elliptical outline, but the AP axis, defined by molecular markers, tends to be perpendicular to the long axis of the ellipse. Subsequently, the relative orientations of the AP axis and of the long axis change so that when gastrulation begins, they are closer to being parallel, albeit not exactly aligned. As a result, most embryos briefly lose their bilateral symmetry when the primitive streak starts forming in the epiblast.
  
  
  CONCLUSIONS
  The change in the orientation of the AP axis is only apparent and results from a dramatic remodeling of the whole epiblast, in which cell migrations take no part. These results reveal a level of regulation and plasticity so far unsuspected in the mouse gastrula.
authors.affiliation: Laboratoire de Développement des Vertébrés, Institut Jacques Monod, UMR 7592 CNRS, Université Paris 6 et 7, 2 place Jussieu, 75251 Paris, France.
authors.forename: Aitana
authors.initials: A
authors.lastname: Perea-Gomez
authors.forename: Anne
authors.initials: A
authors.lastname: Camus
authors.forename: Anne
authors.initials: A
authors.lastname: Moreau
authors.forename: Kate
authors.initials: K
authors.lastname: Grieve
authors.forename: Gael
authors.initials: G
authors.lastname: Moneron
authors.forename: Arnaud
authors.initials: A
authors.lastname: Dubois
authors.forename: Christian
authors.initials: C
authors.lastname: Cibert
authors.forename: Jérôme
authors.initials: J
authors.lastname: Collignon
mesh_terms: D000818:Animals; D019521:Body Patterning / Q000502:physiology*; D004707:Endoderm / Q000502:physiology; D005775:Gastrula / Q000502:physiology* / Q000648:ultrastructure; D018507:Gene Expression Regulation, Developmental / Q000502:physiology*; D006652:Histological Techniques; D051379:Mice; D008822:Mice, Transgenic; D018613:Microscopy, Confocal; D014054:Tomography