title: Mouse primitive streak forms in situ by initiation of epithelial to mesenchymal transition without migration of a cell population.
abstract: BACKGROUND
  During gastrulation, an embryo acquires the three primordial germ layers that will give rise to all of the tissues in the body. In amniote embryos, this process occurs via an epithelial to mesenchymal transition (EMT) of epiblast cells at the primitive streak. Although the primitive streak is vital to development, many aspects of how it forms and functions remain poorly understood.
  
  
  RESULTS
  Using live, 4 dimensional imaging and immunohistochemistry, we have shown that the posterior epiblast of the pre-streak murine embryo does not display convergence and extension behavior or large scale migration or rearrangement of a cell population. Instead, the primitive streak develops in situ and elongates by progressive initiation EMT in the posterior epiblast. Loss of basal lamina (BL) is the first step of this EMT, and is strictly correlated with ingression of nascent mesoderm. Once the BL is lost in a given region, cells leave the epiblast by apical constriction in order to enter the primitive streak.
  
  
  CONCLUSIONS
  This is the first description of dynamic cell behavior during primitive streak formation in the mouse embryo, and reveals mechanisms that are quite distinct from those observed in other amniote model systems. Unlike chick and rabbit, the murine primitive streak arises in situ by progressive initiation of EMT beginning in the posterior epiblast, without large-scale movement or convergence and extension of epiblast cells.
authors.affiliation: Department of Cell Biology, University of Virginia, Charlottesville, Virginia 22908, USA.
authors.forename: Margot
authors.initials: M
authors.lastname: Williams
authors.forename: Carol
authors.initials: C
authors.lastname: Burdsal
authors.forename: Ammasi
authors.initials: A
authors.lastname: Periasamy
authors.forename: Mark
authors.initials: M
authors.lastname: Lewandoski
authors.forename: Ann
authors.initials: A
authors.lastname: Sutherland
mesh_terms: D000818:Animals; D002465:Cell Movement; D058750:Epithelial-Mesenchymal Transition; D005855:Germ Layers / Q000166:cytology / Q000502:physiology; D051379:Mice; D008815:Mice, Inbred Strains; D054240:Primitive Streak / Q000166:cytology* / Q000502:physiology*; D015398:Signal Transduction