Gastrulation - Anatomy & Physiology

Introduction

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

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
Endoderm	Pharynx, lungs, liver, lining of gut

Mechanism of Gastrulation

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

2.Node

3.Tail Bud

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. 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

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

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

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.

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. 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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