Gastrulation in humans

Gastrulation:

Gastrulation is the process of formation of the ‘gastrula’ from the blastocyst. In the gastrula stage, there is a slowing of the rate of cleavage or divisions, but two important events actively take place:

a. Differentiation of blastomeres: This process results in the formation of three germinal layers: ectoderm, mesoderm, and endoderm from the cells of the embryoblast.

b. Morphogenetic movements: These are different types of movements that allow cells to reach their definite places in the embryo.

V. S. of late Gastrula 

• Gastrulation begins in embryoblast cells around the 8th day after fertilization. Hypoblasts, the first layer to differentiate, become flattened and divide, growing towards the blastocoel.
• This layer forms the yolk sac, while epiblasts, the remaining cells, form the primary ectoderm. Both layers form a flat, bilaminar embryonal disc.
• The second layer to differentiate is the ectoderm, where epiblast cells divide and redivide, enclosing the amniotic cavity, which contains the embryonal disc. The yolk sac forms the yolk sac.
• The embryonal ectoderm forms from pyramidal cells in the disc, while amniogenic cells line the amniotic cavity roof.
• These cells divide and redivide to form the amnion, an extra-embryonic membrane that surrounds and protects the embryo.
• The bilaminar embryonic disc is positioned between the amniotic cavity and yolk sac.
• Gastrulation occurs 15 days after fertilization, transforming the disc into a trilaminar one through division, rearrangement, and migration of epiblast cells.
• The process of embryonic development begins with the formation of the primitive streak and primitive groove, which progresses from the posterior to the anterior end of the embryo.
• From this site, a mesoderm layer extends between the ectoderm and endoderm. The anterior end of the groove communicates with the yolk sac via the blastopore.
• The embryonal disc differentiates into three layers: ectoderm, mesoderm, and endoderm.
• The further process after gastrulation is called histogenesis, followed by organogenesis.