Question

Describe the changes involved during gastrulation.

Answer 1

Gastrulation:

The formation of a gastrula from a blastula is known as gastrulation. During gastrulation, three primary germinal layers ectoderm, mesoderm, and endoderm are formed by morphogenetic movements and rearrangement of the cells.  Gastrulation and implantation take place simultaneously.

Gastrulation involves the following changes:
1. Formation of endoderm:
Some cells covering the free surface of the embryonal knob (exposed to the cavity of blastocyst) become flat and form first endodermal cells. Ectodermal cells multiply spread out in all directions and form a complete lining inside the trophoblast of the blastodermic vesicle is called the endoderm. The endoderm located under the embryonal knob is the embryonic endoderm. The remaining endoderm along with the trophoblast forms the primary yolk sac. It encloses a fluid and not the yolk.

2. Formation of embryonic disc:
The blastocyst continues to grow in size. With the growth of the blastocyst, after the formation of the endoderm, the embryonic knob becomes columnar to form an embryonic disc.

3. Formation of amniotic cavity:
Space that appears between the embryonic disc and the trophoblast is called the amniotic cavity. It is filled with amniotic fluid. The roof of this cavity is formed by amniogenic cells derived from the trophoblast and its floor is made up of an embryonic disc.

4. Formation of ectoderm:
The remaining cells of the embryonic disc get arranged in a layer called ectoderm. 
5. Formation of mesoderm:
At the hind end of the embryonic disc, cells start proliferating at an increased rate. The new layer of cells so formed, is the mesoderm, which separates the yolk sac and the amniotic cavity from the trophoblast. 
6. Formation of extraembryonic coelom:
A large cavity called extraembryonic coelom is formed in the extraembryonic mesoderm.

Extra embryonic mesoderm splits this cavity into two layers:
1. Outer mesoderm lining the trophoblast is called the parietal extraembryonic mesoderm.
2. Inner mesoderm covering primary yolk sac and wall of amniotic cavity is termed as visceral extraembryonic mesoderm. 

7. Formation of chorion and amnion:
Chorion consists of a trophoblast lined by parietal extraembryonic mesoderm. Amnion consists of amniogenic cells covered by visceral extraembryonic mesoderm.