Changes

==Gastrulation at the Primitive Streak==

==Gastrulation at the Primitive Streak==

*The epiblast is a simple circular plate of cells.

 

*Cells proliferate and converge through migration, at one pole of the epiblast. These cells become columnar due to space restrictions.

The epiblast is a simple circular plate of cells. Cells proliferate and converge through migration, at one pole of the epiblast. These cells become columnar due to space restrictions. This defines the posterior of the embryo. Eventually, convergence causes cells to be displaced anteriorly. As more cells converge, the primitive streak elongates. Cells in the primitive groove change from columnar to wedge shaped, which produces two elevations either side of a groove. Epiblastic cells of the primitive streak begin to ingress ventrally to form the new germ layers. As the primitive streak extends into the anterior, it enters certain "territories" of the epiblast. These are regions, organised from posterior to anterior, which are destined to become certain germ layers.  

**This defines the posterior of the embryo.

*Eventually, convergence causes cells to be displaced anteriorly. As more cells converge, the primitive streak elongates.

*Cells in the primitive groove change from columnar to wedge shaped, which produces two elevations either side of a groove.  

*Epiblastic cells of the primitive streak begin to ingress ventrally to form the new germ layers.

**As the primitive streak extends into the anterior it enters certain "territories" of the epiblast. These are regions, organised from posterior to anterior, which are destined to become certain germ layers.  

===Endodermal Ingression===

===Endodermal Ingression===

*Cells of the prospective endoderm are the first to ingress.  

 

**Ingression involves the breaking of attachments with neighbouring cells of the epiblast and moving ventrally.  

Cells of the prospective endoderm are the first to ingress. Ingression involves the breaking of attachments with neighbouring cells of the epiblast and moving ventrally. Cells that have broken free of the epiblast make an epithelial to mesenchymal transition. This is achieved by the breaking of the basal lamina, followed by breaking of the intercellular connections. Cells undergoing this transition change from being regularly shaped to being irregularly shaped. When the cells reach the hypoblast, they interchalate with cells of the hypoblast and revert back to their original epithelial state.

*Cells that have broken free of the epiblast make an epithelial to mesenchymal transition.

**This is achieved by breaking of the basal lamina, followed by breaking of intercellular connections. Cells undergoing this transition change from being regularly shaped to being irregularly shaped.

*When the cells reach the hypoblast, they interchalate with cells of the hypoblast and revert back to their original epithelial state.

===Mesodermal Ingression===

===Mesodermal Ingression===

The remaining epiblast forms the ectoderm.

As the streak elongates, it enters the prospective lateral plate mesoderm (LPM) region of the epiblast. Cells of the prospective LPM converge and ingress, making an epithelial to mesenchymal transition. They do not interchalate with the endoderm, but '''remain mesenchymal'''. They then migrate laterally and anteriorly. As the streak extends further, it enters the intermediate and paraxial mesoderm territories, which ingress in a similar fashion. Cells of the LPM are most lateral, then intermediate followed by paraxial mesoderm. As ingression continues and the endoderm continues to interchalate, the hypoblast is pushed laterally. The remaining epiblast forms the ectoderm.

==Gastrulation at the Node==

==Gastrulation at the Node==