5.            GASTRULATION

Cell movements take place during gastrulation as a resulting embryo undergo massive reorganization in turn conversion of a simple spherical ball of cells, the blastula, into a multi-layered organism takes place. At the time of gastrulation movement of a cell from at or near the surface of the embryo to the new and more interior location within the embryo.

During gastrulation formation and organization of primary germ layers called endoderm, mesoderm, and ectoderm into their proper location takes place. The lining of the gut and other internal organs form through endoderm and it is known as the most internal germ layer. Skin, brain, the nervous system, and other external tissues form through ectoderm and it is known as the most exterior germ layer. Muscle, the skeletal system, and the circulatory system form through mesoderm and it is known as the middle germ layer.

In all animal, common cellular mechanisms involved in gastrulation but variation occurs in detailing of gastrulation between various groups of animals. During gastrulation, changes occur within cell motility, cell shape, and cell adhesion. Below are schematic diagrams of the major types of cell movements that occur during gastrulation.

5.1.         Invagination:  Bending of sheets of cells (epithelial sheet) towards inward of the embryo or cell sheet gets infolded into the embryo.

5.2.         Ingression: Epithelial sheet left by the cell’s individually and forms freely migrating mesenchyme cells within embryo or individually, migration of cells into the embryo.

5.3.         Involution: Within embryo inward rolling of an epithelial sheet to form an underlying layer or cell sheet’s inturning above the basal surface of the outer layer.

5.4.         Epiboly: By thinning spreading of cells' sheet or one cell sheet get expended over other cells.

5.5.         Intercalation: Movement of rows of cells between one another, generating an array of cells that is thinner but longer (in one or more dimensions) or formation of the single layer of the epithelial cell through merging of multiple cell layers.

5.6.         Convergent Extension: Intercalation of rows of cells in a highly directional manner.

5.7.         Delimination: Migration and splitting of one cell sheet into two cell sheet.

5.8.         Condensation: Formation of an epithelial structure from loosely pack mesenchyma.

5.9.         Dispersal: Conversion of epithelial cells sheet in to lose mesenchyme.

5.10.      Egression: cells' movement from interior to exterior.

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