Gastrulation - Anatomy & Physiology

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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.

• Ectoderm - outside layer
• Mesoderm - middle layer
• Endoderm - inner layer

Germ Layer	Derivatives
Ectoderm	Neural Ectoderm - PNS & CNS Non Neural Ectoderm - Epidermis of skin
Mesoderm	Axial Mesoderm - Notochord, which is mostly degenerated Paraxial Mesoderm - Somites, which form axial skeleton and dermal muscle of the body Intermediate Mesoderm - Parts of reproductive system and kidneys Lateral Plate Mesoderm - Heart, limbs, tissue around gut
Ectoderm	Parynx, lungs, liver, lining of gut

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 cubiodal - 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:

Primitive Streak

Node

Tail Bud

Gastrulation at the Primitive Streak

• The epiblast is a simple circular plate of cells.
• Cells proliferate and through migration, converge at one pole of the epiblast. These cells become columnar to compensate for the decreased amount of space available.
  • 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 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.
  • 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

• 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 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

• 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 and axial mesoderm.
• As ingression continues, the hypoblast is pushed laterally as the endoderm spreads.