Gastrulation

It is the process of cell and tissue movements whereby the cells of the blastula are rearranged in different manner.

• The blastula consists of numerous cells , the positions are established during cleavage.
• The three germ layers such as ectoderm(outside), endoderm and mesoderm are first produced in the process of gastrulation .
• Skin and nervous system system are spread over its outside surface.
• The movements of gastrulation involve the entire embryo and cell migrations in one part of the gastrulating embryo which  coordinate with other movements occuring simultaneously.

Gastrulation usually involves some combination of the following types of movements -

1. Invagination - (sea urchin endoderm)The infolding of a cells ,much like the indenting of a soft rubber bait when it is pocked .(inward buckling of an epithelium.)

                                Image source credit-https://www.studyblue.com/notes/note/n/developmental-bio-exam-1/deck/5569140

2.Involution - (Amphibian mesoderm) The inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells. It requires migration .(movement of individual cells over other cells ).

                                     Image source credit-http://biology.kenyon.edu/courses/biol114/Chap14/Chapter_14.html

3.Ingression - (Sea urchin mesoderm and Drosophila neuroblasts) Migration of individual cells from the surface layer into the cativy or interior of the embryo .

                             Image source credit -http://www.biology.arizona.edu/developmental_bio/problem_sets/developmental_mechanisms/03t.html

4.Delamination -( Mammalian and bird hypoblast formation ) Spilliting of 1 cell sheet into 2 or more parallel sheets. 

5.Epiboly- (Ectoderm formation in amphibians, sea urchins and tunicates )Spread of an outside cell layer(epidermal sheets) to envelop a yolk mass or deeper layer.or expansion of one cell sheet over other cells.

Types of cell movements during gastrulation 

                                  Image source credit -http://courses.biology.utah.edu/bastiani/3230/DB%20Lecture/Lectures/a7Gast.html


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