Initiation of gastrulation in the mouse embryo is preceded by an apparent shift in the orientation of the anterior-posterior axis.
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.
Perea-Gomez
Aitana
A
Laboratoire de Développement des Vertébrés, Institut Jacques Monod, UMR 7592 CNRS, Université Paris 6 et 7, 2 place Jussieu, 75251 Paris, France.
Camus
Anne
A
Moreau
Anne
A
Grieve
Kate
K
Moneron
Gael
G
Dubois
Arnaud
A
Cibert
Christian
C
Collignon
Jérôme
J
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