Gastrulation class 12th

We are all familiar with the term fertilization and how it takes place, but that's not the end of it. There are more steps that takes place after fetilization like implantation. There is one more step that takes place which is called gastrulation about which we will be talking in this article. 

Related articles: Fertilization in humans

Before we get into gastrulation this a step which comes after implantation which is called blastulation. Not going to keep it long but instead short so it'll be easy to understand it as well as this topic. 

What is blastulation ?

To put it in simple words, blastulation is formation of blastula from a morula. This seems simple when you just define the term but let's see how the process goes. 

The morula has 2 layers which are inner cell mass and outer cells called the trophoblast or trophoectoderm. The inner cells are smaller in size and are called the micromeres and the outer cells are larger in size which are called megameres

The trophoblast cells of the morula absorbs the nutritive fluid which is secreted by the uterine endometrial membrane

In order to form a cavity called the blastocyst or blastocoel or segmentation cavity the cells of the trophoblast has to absorb as much as fluid it can. It becomes flat and hence forms a cavity. The trophoblast is separated form the inner cell mass but only from one side the other side is still attached. 

After the cell mass is detached from one pole the inner cell mass looks like a small knob. This knob which is still attached gives rise to an embryo and is called embrynoic pole or animal pole and the opposite side is called abembryonic pole
Formation of blastocyst

As you can see in the image above, the morula has now formed into a cyst like structure as it has absorbed a lot of the uterine fluid, this morula keeps on expanding in order to form a blastocyst. The fluid is filled in the central space. 

The work of trophoblast which is the outer layer or covering of the blastocyst is done here and it does not help in the formation of the embryo. Instead, it stays on the outside and gives rise to extraembryonic membranes which helps in protecting the embryo and placenta which are required for the nourishment of the embryo. 

The zona pellucida which is a thick transparent membrane which surrounds the ovum becomes thinner and starts to disappear. Zona pellucide doesn't let the implantation of the blastocyst to occur at an abnormal site. As it starts to disappear, it is letting the blastula increase in size and in the volume by 0.15 mm to 0.30mm

The stage of blastulation is reached after five days of fertilization. It totally depends on the mother for nutrition. 

Significance of blastulation:

  1. The fate of the cells are determined in this stage meaning that the you can see the changes happening in the cells. 
  2. When the major organs of the body will be forming they would have no complications as the areas would already be arranged in a pattern in the blastula. 

What is gastrulation ?

Now that we know what blastulation means let move onto gastrulation. Again, in simple words, gastrulation is formation of gastrula from the blastula. Gastrulation occurs after the process of blastulation. 

Formation of gastrula 

In this process, the inner cell mass of the blastocyst starts to separate after the process of implantation forming three primary germ layers - the ectoderm, endoderm and mesoderm which are present in triphoblastic animals. These three layers give rise to the tissues and organs of the adult body and is formed from the inner cell mass. 

To form the germ layers there are some specific movements taking place which are called the morphogenetic movements. The process of gastrulation and blastulation takes place simultaneously.

Changes involved in gastrulation:

Following are the changes that we involved in gastrulation. These changes are in sequential manner:

1. Formation of endoderm:

Endoderm is the first germ layer out to be formed out of the three in the human embryo. This layer is formed by the inner cell mass which gets separated from the trophoblast. The cells sink into the blastocoel to form the endodermal cells. 

The endodermal cells rapidly multiple to form the complete lining in the trophoblast of the blastodermic vesicle which then forms the layer of endoderm. This layer lies below the original outer layers of the cells of blastula. 

When you look at the sectional view of the embryo when in this stage, we see a tube which closes another tube but of a smaller diameter. The inner tube which is of the smaller diameter is called the archenteron which is lined by the endoderm

The cavity of blastocyst or blastocoel is now transformed into the cavity of archentron. The archenteron differentiates into two portions, which are as follows:
  1. One portion which is with the embryo develops in gut tract. 
  2. The other portion which is the remaining endoderm along with the trophoblast which acts as a distal sac communicates with the embryonic gut which forms a yolk sac. 

2. Formation of the embryonic disc:

As the blastocyst keeps growing in size, with this growth and after the formation of endoderm, the embryonic knob becomes columnar to form an embryonic disc. Meaning that the embryonic knob is formed into a column. 

The cells which are arranged in a columnar manner get solid or strong in order to form a embryonic disc. The embryonic disc consist of three regions:
  1. Cephali margin 
  2. Emrbyonic disc proper 
  3. Caudal region

3. Formation of amniotic cavity:

There is a space which appears between the embryonal disc and trophoblast is called the amniotic cavity. This cavity is filled with amniotic fluid. This space takes place due to the separation of the ectodermal cells. 

Amniotic cavity

The amniogenic cells which are derived from the trophoblast forms the roof the amniotic cavity and the embryonic disc forms the floor of the amniotic cavity. 

4. Formation of ectoderm:

Once the endoderm is completed, the remaining cells of the embryonic disc arrange to form a layer of ectoderm

The ectoderm is formed from the outer cells of the inner cell mass that becomes columnar and this later is called as the columnar epithelium. The ectodermal cells closes the cavity of the yolk sac as this does not function in humans. 

Primitive streak starts to appear in an oval area which is towards the posterior (back) end of the cells and forms a bulge in the amniotic cavity. As the embryonic dics keeps getting longer so does the primitive streak. 

5. Formation of mesoderm:

Once the endodermal layers are in place, the cells present at the caudal end of embryonic disc starts to multiply rapidly. This gives an increase in the thickness of the dics which later gets separates from the embryonic dics giving rise to a layer of mesoderm. 

Mesoderm is basically formed by the multiplication and division of the cells which takes place in the region of the primitive streak. This multiplication and division happens sideways between the ectoderm and the endoderm. 

6. Formation of extra embryonic coelom:

A cavity is formed in the embryonic mesoderm which is called the extraembryonic coelom. This mesoderm is called extraembryonic because it lies on the outside of the embryonic dics. Extra embryonic mesoderm splits the cavity into two parts, which are as follows:
  1. The outer mesoderm lining of the trophoblast which is called the parietal extra embryonic mesoderm. 
  2. The inner mesodorm which is covering primary yolk and the wall of amniotic cavity is called the visceral extra embryonic mesoderm. 

7. Formation of chorion and amnion:

Chorion consist of trophoblast which is lined by the parietal extraembryonic mesoderm and the amnion is consist of amniogenic cells covered by the visceral extraembryonic mesoderm

Significance of gastrulation:

Gastrulation carriers great significance with it. Without the process of fertilization wouldn't even be completed. So, here are few significance of gastrulation:
  • The cells of blastula is rearranged by morphogenetic movements which results in the formation of the three germinal layers - Ectoderm, Endoderm and Mesoderm. 
  • It slows down the rate of cleavage.
  • There is an increase in the metabolic activities. 
  • It is the beginning of morphogenesis and differentiation. 

Fate of three germinal layers:

When all the three germinal layers are formed, the process of gastrulation is said to be completed. After the formation of these three germinal layers they give to the main organs/tissues of the human body and formation of these tissues or organs if know as histogenesis

Here is a table showing the organs that are formed these three germinal layers:

Ectoderm Endoderm Mesoderm
Gives rise to - epidermis of skin, epidermal derivaties like hair, nails, sweat glands, conjunctiva, cornea, lens, retina, internal and external ear, enamel of teeth, nasal cavity, adrenal medulla, stomodaeum proctodaeum, pituitary gland, nervous system (brain, spinal cord and nerves), pigment cells, visceral and cranial cartilage (partly), buccal cavity, lower part of anal canal, salivary glands and cutaneous gland. Gives rise to - epithelium of midgut (pharynx to colon), glands of stomach, intestine, tongue, tonsils, lungs, trachea, bronchi, larynx, unrinary bladder, vagina in females, liver, pancreas, anterior lobe of pituitary gland, thyroid gland, parathyroid, thymud, eustachian tube, urehra in males, gastric glands, epithelium of digestive tube and germ cells. Gives rise to - dermis of skin, muscles of the body, connective tissues, cartilage bone and notochord, epithelium blood vessels and blood, bone marrow, lymphatic system, kidneys, ureters, gonds, genital ducts, circulatory system including heart, adrenal cortex, middle ear, dentine of teeth, vertbral column and appendicular skeleton, urinary ducts, spleen and coelmic epithelium

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