Editing Gastrulation (section)

===Symmetry breaking===
In preparation for gastrulation, the embryo must become asymmetric along both the [[Anatomical_terms_of_location#Proximal_and_distal|proximal-distal axis]] and the [[Anatomical terms of location#Axes|anteroposterior axis]]. The proximal-distal axis is formed when the cells of the embryo form the "egg cylinder", which consists of the extraembryonic tissues, which give rise to structures like the [[placenta]], at the proximal end and the [[epiblast]] at the distal end.  Many signaling pathways contribute to this reorganization, including [[bone morphogenetic protein|BMP]], [[fibroblast growth factor|FGF]], [[nodal signaling|nodal]], and [[Wnt signaling pathway|Wnt]]. Visceral endoderm surrounds the [[epiblast]]. The [[Anatomical terms of location#Proximal and distal|distal]] visceral endoderm (DVE) migrates to the [[anterior]] portion of the embryo, forming the [[anterior visceral endoderm]] (AVE). This breaks anterior-posterior symmetry and is regulated by [[NODAL|nodal]] signaling.<ref name=Arnold />
[[File:Epithelial–mesenchymal transition scheme.png|thumb|right|[[Epithelial–mesenchymal transition]] – loss of cell adhesion leads to constriction and extrusion of newly formed [[mesenchymal]] cell.]]