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Gastrulation - Anatomy & Physiology (view source)

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*Cells in the primitive groove change shape 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.

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**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.

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===Endodermal Ingression===

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

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

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

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**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.

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

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epithelial state.

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===Mesodermal Ingression===

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*As the streak elongates it enters the prospective lateral plate mesoderm (LPM) region of the epiblast.

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*Cells of the prospective LPM converge and ingress, making an epithelial to mesenchymal transition.

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*They do not interchalate with the endoderm, but '''remain mesenchymal'''.

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**They then migrate laterally and anteriorly.

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*As the streak extends further, it enters the intermediate and paraxial mesoderm territories, which ingress in a similar fashion.

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*Cells of the LPM are most lateral, then intermediate followed by paraxial mesoderm and axial mesoderm.

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*As ingression continues, the hypoblast is pushed laterally as the endoderm spreads.

Shamilton

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