Major Cell Migrations During Animal Development:

During gastrulation, cell migration helps to massively reorganize the embryo to form a spherical ball of cells made of three germ layers. Cell migration during development is called morphogenetic movements and is irreversible. The different types of cell migrations during animal development are explained below:


In emboly, sheets fold or invaginate.

The sheet may migrate into a cell aggregate or form beneath another layer.

The three types of embolic movements are invagination, involution, and ingression.


Invagination or in pocketing occurs during gastrulation.

It occurs mostly in the vegetal pole.

During invagination, the exterior sheet of the cells folds towards the interior region.

It is the sinking of a layer of cells.

The number of cells in the blastula determines the complexity of the invagination.

Invagination establishes the body plan.

Invagination occurs either axially and orthogonally.

Axial invagination forms at a single point along the axis of the cell surfaces.

Orthogonal invagination occurs linearly and trough-like.

Example: Amphioxus gastrulation and tunicate gastrulation.


Involution is the shrinking of the cells.

The turning in of the expanded layer of cells is called invagination.

The surface layer of cells can turn inside itself and continue to expand on the inner surface of the superficial layer.

Cell away from the site of invagination move to the margin of the site of invagination. An example is the Lip of the blastopore during gastrulation.

Proteolysis of the basement membrane causing epithelial regression and apoptosis.

This reduces the size of the structure.

Involution occurs during limb development in some chordates.

Involution also occurs during the atrophy of the thymus during puberty.


Ingression is an internalizing cell movement.

It is also called polyinvagination.

Small groups of independent cells slough off from the epithelial layer and migrate to blastocoel or another embryonic cavity. An example is the formation of the primitive streak in amniotes.

It changes the relative position of the cells.

During ingression, the epithelial cells held by tight junctions are relieved and transforms into freely migration mesenchymal cells.

Neural crest cells and sea urchin gastrulation are other examples of ingression.

During epithelial to mesenchymal transition (EMT), the primary mesenchymal cells detach from the epithelial cells by breaking the tight junctions and forms freely migrating secondary mesenchymal cells.

During ingression, the adhesion properties of the cells are changed.

In bird and mammalian development, the epiblast cells converge at the midline and ingress at the primitive streak forming the mesoderm.


During delamination cells that form a single sheet splits and forms two sheets.

A mass of cells separates from the epithelial layer and forms an inside sheet.

Sometimes the division of cells in a plane parallel to the tissue space may also occur.

During delamination cells lose the adhesion junctions and separate from the epithelium.

An example is neural crest delamination.


Epiboly occurs during gastrulation.

The word is derived from the Greek word meaning throwing on or extending upon.

During epiboly, the epithelial cells are spread across. When a layer of epithelial cells extends towards a single point on the surface, it is called the convergence.

Epiauxesis is a type of epiboly that occurs in zygotes that undergo spiral cleavage.

Epiauxesis occurs in the absence of hyaline membrane, absence of a surface for attachment of blastomere, mitotically active, and absence of cell adhesion. In epiauxesis, overgrowth of micromeres over macromeres occurs.

Initially, a single monolayer of cells changes physically in terms of shape.

In some types, the cells first undergo intercalation, followed by a physical change of the cells.

Epiboly is seen in sea urchins, tunicates, amphibians.

Major Cell Migrations During Animal Development:
Concrescence: In concrescence, several cells move towards each other and fuse. Examples are the development of feathers in birds.

Divergence: It is the opposite of convergence where cells that involuted over the blastopore during gastrulation, migrate and diverge to their future positions. Examples are lateral plate and ventral mesoderm in amphibians, lateral plate, and extraembryonic mesoderm in reptiles, birds, and mammals.

Extension: During extension, the gastrula elongates anterio-posteriorly. Examples include elongate of presumptive neural and epidermal areas in chordates from tunicates to mammals.

Cell proliferation: During the gastrulation of hemichordates and amphibians, cell proliferation occurs.

Evagination: It is the opposite of the invagination where cells migrate from the interior to the exterior where out pocketing of cells occurs.

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