Notochord | Embryology | Anjani Mishra

Hey, In this post you are going to know about the notochord and some of its functions with the formation of the notochord.

So, To know about notochord, let's start with the definition of the notochord

Definition: A notochord is a small flexible rodlike cord derived from mesenchymal cells of mesoderm that appear as the forerunner of the vertebral column in phylum Chordata.  

What is notochord made of?

Notochord is mainly made from the mesenchymal cells of mesoderm which is a part of the trilaminar germ disc that appears between ectoderm and endoderm.

What is the function of the notochord?

The main functions of the notochord is as follows;

Notochord primarily helps in the formation of the notochordal canal.

Notochord through this canal makes a communication between the amniotic cavity above and the yolk sac below.

Notochord through this communication provides nutritional supply to the germ disc.

Notochord appears as the forerunner of the vertebral column.

Notochord with the formation of the vertebral column helps in the lodgement of the spinal cord in the vertebral column. 

However, with the formation of the vertebral column, the notochord disappears but it appears as nucleus pulposus of an intervertebral disc.

Do you know what is an intervertebral disc?

An intervertebral disc is a fibrocartilagenous structure which attaches cranial and caudal extremities of adjacent vertebrae and are usually convex and concave, respectively.

Formation of Notochord

If you want to know about the formation of the notochord, you must know about the formation of germ disc first, i.e., the formation of bilaminar and trilaminar germ disc.

So, the question is that how bilaminar and trilaminar germ disc is formed?

Let’s know about the formation of bilaminar germ disc in short. 

Bilaminar germ disc

Bi means two or double and laminar means layer means a double-layer disc-like embryonic structure.

Bilaminar germ disc is formed from the nner cell mass or embryoblast that differentiate into two layers, i.e., hypoblast cell layer below and epiblast cell layer above.

What is the hypoblast cell layer?

The answer is that, it is a layer of small cuboidal cells adjacent to the blastocyst cavity is known as the hypoblast cell layer. It is located below the epiblast cell layer that gives rise to the endoderm.

What is the epiblast cell layer?

The answer is that it is a layer of high columnar cells adjacent to the amniotic cavity is known as the epiblast cell layer. It is located above the hypoblast cell layer that gives rise to the ectoderm.

In this way, the formation of ectoderm from the epiblast cell layer and endoderm from the hypoblast cell layer are jointly known as bilaminar germ disc.

For detail, you can follow the last post on my website or click here.

Now, I think after knowing all about the bilaminar germ disc, it will be easier for you to understand about the formation of a trilaminar germ disc.

Tri-laminar germ disc

Tri means three or triple and laminar means layer means a triple-layer disc-like embryonic structure.

As you got to know that the epiblast cell layer is responsible for the formation of ectoderm.

Further, the ectodermal layer differentiates into three functional zones, i.e., surface ectoderm, neural plate, and pluripotent cellular zone.

Among this, the pluripotent cellular zone is mainly responsible for the further development of the trilaminar germ disc.

Initially the pluripotent cells migrate towards the primitive streak. During this migration they detach from the epiblast and invaginate beneath it between epiblast (ectoderm) and hypoblast (endoderm) forming mesoderm.

In this way the tri-laminar germ disc, i.e., ectoderm, mesoderm and endoderm are formed.

It must be kept in mind that, what I have stated above is not sufficient for you to understand for the first time, but I'm trying to recall you for better understanding.

Remember that the subject is Embryology or Developmental Anatomy means developing form and structure at every moment from fertilization till adulthood.

Actually, this subject is like a story or serial watching on television. If you miss one episode, you can’t understand the next episode. So, it is essential for you to know what happens before and after.

Now I think you will be able to understand the facts, especially the facts about developing Anatomy. Really it is very interesting to know what happens before and what next?

My suggestion is that if you want to know more detail go through my serial post or click here.

How notochord is formed?

The cephalic end of the primitive streak becomes swollen by the proliferation of cells forming a primitive knot or Henson’s node and the depressed part appears at the center of the primitive knot, known as primitive pit or blastopore.

To know about notochord formation, you must know about the cephalic end and primitive streak.

So, what is the cephalic end?

The head end of the future axis of an embryo which is somewhat broader than the caudal end is known cephalic end.

What is primitive streak?

As you know that three functional zones are differentiated in the ectodermal layer of germ disc. 

Among which pluripotent cellular zone which consists of a group of fast proliferating cells accommodated at the caudal end, differentiate very rapidly and form a linear opacity in the midline called primitive streak.

The primitive pit continues forward and downward and the cells surrounding the primitive pit extend gradually in the form of a solid tube cranially along the midline between ectoderm and endoderm up to the prechordal plate.

What is prechordal plate?

It is a plate-like structure of endoderm located on the tip of the notochord and is in close contact with the ectoderm. It is somewhat swollen and gives rise to the cranial mesoderm. 

The cavity of the primitive pit also extends through the cellular extension forming a canal known as the notochordal canal. 

Subsequently, the floor of the canal breaks down and communicates with the yolk sac below and amniotic cavity above through primitive pit.

What is the yolk sac? 

It is a balloon-like a small cavity that appears below the hypoblast. It is a part of the fetal membrane which is not included within the body of the embryo.

How yolk sac is formed?

Yolk sac is formed by the hypoblast cell layer when unilaminar blastocele is encircled by hypoblast forming bilaminar blastocele that gives rise to the primary yolk sac. 

What is amniotic cavity?

It is also a balloon-like a small cavity that appears above the epiblast. It is a part of the fetal membrane which is not included within the body of the embryo.

How the amniotic cavity is formed?

Amniotic cavity is formed by the epiblast cells adjacent to the cytotrophoblast cells of outer cell mass, are jointly called amnioblast. It consists of amniotic fluid.

Therefore at this stage the yolk sac and the amniotic cavity becomes continuous and probably this occurs to supply nutrition to the ectodermal layer of germ disc.

Gradually the notochordal wall becomes flat to form a notochordal plate and this plate forms a fold along the long axis at the roof of the yolk sac which is now lined by the endodermal layer. 

This fold becomes deep to form a tube and this tube is converted into a solid chord of cells which is known as the definitive notochord.

The notochord acts as a forerunner for the development of the vertebral column. The notochord becomes elongated and gets separated from the endoderm and takes a midline position.

What is vertebral column?

The vertebral column is the a fundamental part of the axial skeleton consists of a chain of median, unpaired, and irregular bones that extend from occipital condyle to the end of a tail.

However, with the formation of the vertebral column, the notochord disappears but parts of it persist as pulposus of intervertebral disc.


Conclusion:  

I hope this guide helped you to understand the notochord and its functions and formation. 

If it helped you leave a comment below,

What is notochord?

A notochord is a small flexible rodlike cord derived from mesenchymal cells of mesoderm that appear as the forerunner of the vertebral column in phylum Chordata.

How notochord is formed?

The cephalic end of the primitive streak becomes swollen by the proliferation of cells forming a primitive knot or Henson’s node and the depressed part appears at the center of the primitive knot, known as primitive pit or blastopore.

The primitive pit continues forward and downward and the cells surrounding the primitive pit extend gradually in the form of a solid tube cranially along the midline between ectoderm and endoderm up to the prechordal plate.

The cavity of the primitive pit also extends through the cellular extension forming a canal known as the notochordal canal. 

Subsequently, the floor of the canal breaks down and communicates with the yolk sac below and amniotic cavity above through primitive pit.

Therefore at this stage the yolk sac and the amniotic cavity becomes continuous and probably this occurs to supply nutrition to the ectodermal layer of germ disc.

Gradually the notochordal wall becomes flat to form a notochordal plate and this plate forms a fold along the long axis at the roof of the yolk sac which is now lined by the endodermal layer. 

This fold becomes deep to form a tube and this tube is converted into a solid chord of cells which is known as the definitive notochord.

The notochord acts as a forerunner for the development of the vertebral column. The notochord becomes elongated and gets separated from the endoderm and takes a midline position.

However, with the formation of the vertebral column, the notochord disappears but parts of it persist as pulposus of intervertebral disc.

If you have any questions you can ask me on :

mishravetanatomy@gmail.com 

Facebook Veterinary group link -  https://www.facebook.com/groups/1287264324797711/ 

Twitter - @MishraVet
Facebook - Anjani Mishra

Website: mishravetanatomy.blogspot.com