Gastrulation is the process in which the embryoblast acquire the shape of a disc and becomes tri laminar. Tri laminar means three layers. From superficial to deep these layers are ectoderm, mesoderm, and endoderm. It is very important for the students to understand the formation of three germ layers because of all the cells, tissues, and organs are derived from these layers.

Gastrulation is the most characteristic event that occurs during the third week of development.

Three germ layers formation

First, the embryoblast differentiates into layers, a superficial layer called endoderm and a deep layer comprising columnar cells called ectoderm.

The ectoderm forms a linear thickening in the midline of the embryonic disc called a primitive streak.

The primitive streak then gives rise to the third layer called mesoderm that is situated between the ectoderm and endoderm. The embryo at this time is called a tri-laminar disc.

Through the formation of three germ layers, other embryonic structures also develop.

Besides the three germ layers, what are the other structures formed during gastrulation?

The other structures that are formed during gastrulation are

1. Formation of the amniotic cavity
A little cavity appears between the ectoderm and trophoblast that develops the amniotic cavity. Few cells of the trophoblast delaminate and form the roof of the amniotic cavity. The angiogenic cells produced from the trophoblast secrete amniotic fluid within the amniotic cavity.

2. Formation of the yolk sac
The cells of the endoderm proliferate and range the cavity of the blastocyst. The blastocele is now called the principal yolk sac. The cells that lined the yolk sac form the Heuser membrane.

3. Formation of  extraembryonic mesoderm 
A mass of the trophoblastic cells split the amniotic cavity and yolk sac from the trophoblast. This cell mass is called the extraembryonic mesoderm.

4.  Formation of extraembryonic celom
A number of small cavities appear in the extraembryonic mesoderm later join and form the extraembryonic celom. Because of the formation of extraembryonic celom, the yolk sac becomes smaller and is called the secondary yolk sac.

Because of the extraembryonic celom, the extraembryonic mesoderm splits into two layers. The first layer coats the trophoblast and is called a somatopleuric layer of the extraembryonic mesoderm. The second layers coat the yolk sac and are called the splanchnopleuric or visceral layer of the extraembryonic mesoderm.

5.  Formation of connecting stalk
The extraembryonic celom doesn’t extend to the cranial part of the extraembryonic mesoderm that lies between the amniotic cavity and trophoblast. This part of the extraembryonic mesoderm forms the connecting stalk.

6.Formation of Chorion
Chorion consists of a somatopleuric layer of extraembryonic mesoderm and trophoblast covering it.

7.Formation of amnion
The amnion consists of amniogenic cells and a somatopleuric layer of the extraembryonic mesoderm.

8. Formation of prochordal plate
At one end of the embryonic disc, a rounded area becomes thicker and curved. This curved area is called the prochordal plate. Around the prochordal plate, the cuboidal endodermal cells become columnar. After the development of the prochordal plate, the cranial and caudal end of the embryo are identified. The end at which the prochordal plate appears is called the cranial end while the opposite end is called the caudal end. This plate also determines the central axis.

9.Formation of the primitive streak
At the start of the third week of development, a longitudinal ridge appears in the midline at the caudal end of the embryonic disc. This ridge is called the primitive streak. The primitive streak is formed because of the proliferation of the ectodermal cells.

In the cranial end of the primitive streak, the cells proliferate and form a curved elevation called primitive node or Henson’s node.

10. Formation of the notochord
A depression in the center of the primitive node calls blastopore. A layer of cells develops cranially from the underneath of the blastopore between the ectoderm and mesoderm up to the prechordal plate and form the notochord. The notochord is a very important structure that plays a major role in the prenatal development of the human nervous system.

11. Formation of intraembryonic mesoderm
The cells of the primitive streak invaginate to the endoderm and form a primitive groove. From the bottom of this groove, the cells of the primitive streak spread in between the endoderm and ectoderm and form the intraembryonic mesoderm which is the third germ layer.

The intraembryonic mesoderm spread to all the regions except three regions
1. Region if the prochordal plate
2.  The spot of the notochord
3.  Septum transversum.

What is the modern concept of gastrulation?

According to the modern concept of gastrulation, all three germ layers are derived from the epiblast.

1. The cells of embryo last differentiate into two layers that are the epiblast and hypoblast. The epiblast is the superficial layer while the hypoblast is a deep layer.

2. The cells of the epiblast migrate in the primitive streak and slide beneath it.

3.  Few of the cells replace the hypoblast cells and form the endoderm.

4. Another layer of cells migrate in the primitive streak and laid between the endoderm and epiblast and form the mesoderm.

5. The rest of the cells of the epiblast form the ectoderm.

What are the derivatives of the germ layers?

The sense organs are created by the ectoderm. the musculoskeletal system, cardiovascular system, and the majority of the urogenital system are created by the mesoderm. The coating of the gastrointestinal tract, respiratory tract, and urinogenital tract is formed by the endoderm.

What is the effect of the dominance of 1 layer over the other two layers?

The dominance of one layer in the three layers can change the morphology of the body in postnatal life. There are three types of dominance.

1. Ectomorphic. Ectomorphic is ectodermal dominance. Individuals in which the ectomorphic dominance occurs are long and thin.

2. Endomorphic. Endomorphic is endodermal dominance. Individuals in which the isomorphic dominance occurs are short and thick.

3. Mesomorphic. Mesomorphic is mesodermal dominance. Individuals in which the mesomorphic dominance occurs are well-built means intermediate between the higher two types.