Review of 1st week (Blastulation) – Embryology

So in this article we will talk about “Blastulation” .

Before diving into the “blastulation” , I want you to be aware of the term – “Morula” , most of you must be aware of “Morula” but for those who are not aware , let me give you a review of intial stages of embryo , as I will be starting Embryology from the blastulation , I will not be talking about the stages from fertilization upto morula but trust me not learning those process deeply won’t effect your Core Concept clarity regarding embryology

Morula

As you might be knowing that the Zygote undergo division to form more and more cells it goes from 1 to 2 cells , than divide to form 4 cells , than again divide to form 8 cells and process goes on . As you can see in below attached image.

When the embryo reaches to a 16 celled stage , it forms a solid ball of cells which gives appearance like “Mulberry”, and at that stage we call that solid ball of cells surrounded with the zona pellucida as – “Morula”, see the given image below.

So,Morula is solid ball of cells having 16 (or more) cells inside it , yes the cells are not just limited to 16 cells until the ball appear like mulberry and the cells inside it are not specialized , it will be called as – “Morula” and the cells which are not specialized are known as “Totipotent cells” . Totipotent cells can take any form they want , i mean any cell from those can form any organ , if you damage the growing embryo at this stage , suppose you pricked one of the cell with a microneedle , it will not effect the growing embryo in any way as the damaged cell will be replaced by some other cell by divison and they were not specialized at that time so there would be no abnormality in the grown up baby as the cells which will take it place will do its work .

So Remember Morula is 16+ solid ball cell mass having totipotent character ,the cells of morula are known as -“Blastomeres”. So you can say “Morula is solid ball of 16+ solid ball of blastomeres”

Note :- In initial stages when growing embryo reach to 16 blastomere stage, they are not specialized but once they reach this stage the cell undergo specialization slowly and that marks the starting of a process – ” Blastulation”

Specialization of blastomeres

Now the morula undergo change , the blastomeres start specializing .

Changes that occur:-

• The blastomeres move toward periphery and stick there

I told you that morula is a solid ball of cells but as the process of blastualtion begins the cells start to migrate toward the periphery making a hollow ball of cells.

Think it like a plastic ball which is empty from inside , just like that the blastomeres start collecting toward peiphery forming a empty cavity inside called – Blastocoel.

Now after this ,there occurs an another change, any cells group from periphery ,Note again- any group of cells which are acting to make blastoderm divide to make cluster of cells on inner side (in the blastocoel), and the cells which are formed on inner side are collectively called as – “Inner cell mass ” , and the ouyter cells which were located at periphery are called as – “outer cell mass ” or Blastoderm .see the image attached below:-

see the above image . Now the cells are specialized and so no longer be known as blastomeres , we call the group of cells on outer side forming the peripheral layer as – “outer cell mass” and the cluster of cells on inner side formed from the division from outer cell mass as -“Inner cell mass”

Note :- The cells have now become specialized and damage at these developing stages will cause an defect .

The outer cell mass layer is called as – Trophoblast and the cluster of inner cell is simply called as – “Inner cell mass”.

Real pic :-

and at this stage – the ball of cells with the outer trophoblast and inner cell mass with blastocoel cavity is actually known as “blastocyst” and this stage marks the end of process called Blastualtion (Formation of blastocyst) . one last thing i want you to remember is that – zona pellucida is still presesnt upto this stage and Blastocyst is formed at 5th day after fertilization.

Review images :-

I will continue the embryology series and next time i will be coming with gastrulation so make sure you stay updated and do comment below that how was the content , response from your side really make me work more harder.

Signing off —

Bhavesh saini