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==Introduction==

==Introduction==

Gastrulation is the process of forming the three germ layers; '''ectoderm''', '''mesoderm''' and '''endoderm'''. It is achieved through a series of highly coordinated series of cell movements. Cells that will form the endodermal and mesodermal organs are brought inside the embryo, whilst cells that will form ectoderm move to spread out over the outside of the embryo.

Gastrulation is the process of forming the three germ layers; '''ectoderm''', '''mesoderm''' and '''endoderm'''. It is achieved through a series of highly coordinated cell movements. Cells that will form the endodermal and mesodermal organs are brought inside the embryo, whilst cells that will form ectoderm move to spread out over the outside of the embryo.

'''Ectoderm''' - outside layer

'''Ectoderm''' - outside layer

'''Endoderm''' - inner layer

'''Endoderm''' - inner layer

==Mechanism of Gastrulation==

==Mechanism of Gastrulation==

When the embryo is in it's blastula stage it has two cell layers; the epiblast and hypoblast. The '''epiblast''' is columnar, whilst the '''hypoblast''' is cuboidal. Each is a single cell thick. The epiblast gives rise to all three cell layers; the hypoblast makes no contribution. The endoderm and mesoderm develop by proliferation and migration of cells of the epiblast, what remains of the epiblast forms the ectoderm. The epiblast is a circular plate of cells. In gastrulation, cells ingress ventrally from the epiblast to form the three germ layers. Gastrulation occurs at three embryonic sites:

When the embryo is in its blastula stage it has two cell layers; the epiblast and hypoblast. The '''epiblast''' is columnar, whilst the '''hypoblast''' is cuboidal. Each is a single cell thick. The epiblast is a circular plate of cells that ingress ventrally to give rise to all three cell layers. The hypoblast makes no contribution. The endoderm and mesoderm develop by proliferation and migration of cells of the epiblast, what remains of the epiblast forms the ectoderm.  

 

Gastrulation occurs at three embryonic sites:

1.'''Primitive Streak'''

1.'''Primitive Streak'''

===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 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 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 intercalate with cells of the hypoblast and revert back to their original epithelial state.

===Mesodermal Ingression===

===Mesodermal Ingression===

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.

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 intercalate 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 intercalate, the hypoblast is pushed laterally. The remaining epiblast forms the ectoderm.

==Gastrulation at the Node==

==Gastrulation at the Node==

Cells at the posterior of the primitive streak are allocated to form the tail bud. When the primitive streak elongates, these cells don't follow, but migrate posteriorly and form a condensed ball of cells called the tail bud. The tail bud exists to form cells of the hindlimb. Tail notochord and somites form through regression of the tail bud. No endoderm exists in the tail bud. Production of the tail bud is called '''secondary gastrulation''', production of the primitive streak, node and notochord is called '''primary gastrulation'''. Without the tail bud, the notochord would run out of cells before the hindlimb could be made. In animals that have tails, cells at an additional site undergo gastrulation.

Cells at the posterior of the primitive streak are allocated to form the tail bud. When the primitive streak elongates, these cells don't follow, but migrate posteriorly and form a condensed ball of cells called the tail bud. The tail bud exists to form cells of the hindlimb. Tail notochord and somites form through regression of the tail bud. No endoderm exists in the tail bud. Production of the tail bud is called '''secondary gastrulation''', production of the primitive streak, node and notochord is called '''primary gastrulation'''. Without the tail bud, the notochord would run out of cells before the hindlimb could be made. In animals that have tails, cells at an additional site undergo gastrulation.

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