Changes

==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

| '''Ectoderm'''

| '''Ectoderm'''

|'''Neural Ectoderm''' - PNS & CNS

|'''Neural Ectoderm''' - PNS & CNS

'''Non Neural Ectoderm''' - Epidermis of skin

'''Non Neural Ectoderm''' - Epidermis of [[Skin - Anatomy & Physiology|skin]]

|-

|-

| '''Mesoderm'''

| '''Mesoderm'''

'''Paraxial Mesoderm''' - Somites, which form axial skeleton and dermal muscle of the body

'''Paraxial Mesoderm''' - Somites, which form axial skeleton and dermal muscle of the body

'''Intermediate Mesoderm''' - Parts of reproductive system and kidneys

'''Intermediate Mesoderm''' - Parts of [[Reproductive System Overview - Anatomy & Physiology|reproductive system]] and [[Renal Anatomy - Anatomy & Physiology|kidneys]]

'''Lateral Plate Mesoderm''' - Heart, limbs, tissue around gut

'''Lateral Plate Mesoderm''' - [[Heart - Anatomy & Physiology|Heart]], limbs, tissue around [[Alimentary System Overview - Anatomy & Physiology#Small Intestine|gut]]

|-

|-

| '''Ectoderm'''

| '''Endoderm'''

|Parynx, lungs, liver, lining of gut

|Pharynx, lungs, liver, lining of gut

|}

|}

*When the embryo is in it's blastula stage it has two cell layers; the epiblast and hypoblast.

3.'''Tail Bud'''

::'''Tail Bud'''

==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 intercalate 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===

*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.

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.

*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.

==Gastrulation at the Node==

==Gastrulation at the Node==

*The primitive streak stops elongating at about 75% of the way along the length of the posterior - anterior axis.  

The primitive streak stops elongating at about 75% of the way along the length of the posterior - anterior axis. Cells condense at this anterior point of the primitive streak and continue to proliferate; called '''Hensen's Node'''. They then ingress and replace the anterior hypoblast and form foregut endoderm. Cells then migrate anteriorly, leaving the node. These cells will form all of the head mesoderm. The node then regresses posteriorly, destroying the primitive streak and forming the '''notochord''' (axial mesoderm). The notochord begins at the level of the midbrain. Notochord regression also produces paraxial mesoderm. Anterior cells are developing before posterior cells have undergone gastrulation.

**Cells condense at this anterior point of the primitive streak and continue to proliferate.

**They then ingress and replace the anterior hypoblast and form foregut endoderm.

**Cells then migrate anteriorly, leaving the node. These cells will form all of the head mesoderm.

***The node then regresses posteriorly, destroying the primitive streak and forming the '''notochord''' (axial mesoderm).

***The notochord begins at the level of the midbrain.

****Anterior cells are developing before posterior cells have even undergone gastrulation.

==Gastrulation at the Tail Bud==

==Gastrulation at the Tail Bud==

*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.

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.

*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 primitve 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.