What is Cleavage and Implantation ? Embryology | Anjani Mishra

What is Cleavage & What is Implantation ?

Know about Cleavage and Implantation

In this post you are going to know about what is cleavage and implantation.

But to know about cleavage and implantation, you first must know about ovulation and fertilization.

So what is ovulation ?

Ovulation is the process of rupture of mature follicle and discharge of ovum from the ovary is known as ovulation.

Now,  you know about what is ovulation, So you must also know about what is fertilization.

What is fertilization ?

Ferlization is the process of fusion of two mature germ cells, an ovum and spermatozoan to form a mono-nucleated single cell, the zygote.

For more information about ovulation and fertilization, You can head to my last post or click here :- Ovulation and Fertilization

Now, Lets head to What is cleavage ? 

What is Cleavage ?

Fig: Cleavage division

Now lets start to know about cleavage 

 

To know about cleavage we need to know about it's Definition first.

Definition: The series of cell division that takes place immediately after fertilization is called cleavage. 

These daughter cells, which becomes smaller with each cleavage division, are known as blastomeres.

Once the zygote has reached the two-cell stage, it undergoes a series of mitotic cell divisions, resulting in a rapid increase in the number of cells.

Do you know, What is zygote ?

The mono-nucleated single cell fertilized ovum is known as zygote.

What is Morula (mulberry)

After 3 to 4 divisions the embryo, i.e., the blastomere cells increase in number at certain stage, [approximately 3 days after fertilization] where the cells are 16 in number and looks like a mulberry fruit is called morula stage.

What does morula consists of ? 

The morula cousists of a group of centrally located cells, the inner cell mass which gives rise to the development of embryo proper while surrounding cells of morula compose the outer cell mass, that forms the traphoblast which later contributes to the placenta. At this stage the embryo is about to enter the uterus. 

Fig: Showing inner cell mass and outer cell mass

Do you know how blastocyst is formed ?

Formation of blastocyst

When morula enters the uterine cavity, fluid begins to penetrate through the zona pellucida into the inter-cellular spaces of the inner cell mass.

Gradually, the intercellular spaces become confluent, and finally, a single cavity, blastocele is formed. 

At this stage, the embryo is known as blastocyst/blastula stage.

Cells of inner cell mass, now referred to as the embryoblast, are located at one pole, while the outer cell mass or trophoblast becomes flattened and form the epithelial wall of the blastocyst. 

What is Embryoblast ?

The inner cell mass that gives rise to the development of future axis of embryo is called embryoblast.

The zona pellucida now disappeared, allowing implantation to begin.

Now, You already knew about what is cleavage.

So, Let's know about what is implantation and it's process.

What is Implantation?

Fig: Implantation 

Difinition : Implantation is the process of attachment of blastocyst with or within the endometrium of uterus. 

Zona pellucida disappears at the end of 5^th day of fertilization.

Implantation or embedding of the blastocyst takes place on the 6^th or 7^th day after fertilization.

Implantation is initiated by the traphoblast cells, located over the embryoblast pole. 

What is trophoblast ? 

The outer cell mass of morula that contribute to form future placenta is called trophoblast.

The trophoblasts are capable of secreting digestive/proteolytic enzyme that dissolves the zona pellucida and cause penetration and subsequent erosion of epithelial cells of the uterine mucosa. 

The uterine mucosa, however, promotes the proteolytic action of the blastocyst, so that the cells of embryo can receive nutrition and oxygen from uterine milk secreted by the uterine glands. 

The sticky nature of the milk help the embryo to settle in the uterine pits and the process of implantation begins for further development.

In this way by the end of 1^st week of development, the zygote has passed through the morula and blastula stages and has begun implantation in the uterine mucosa.

Formation of germ layer

Bilaminar germ disc:

(2^nd week of development) 

Fig: Bilaminar germ disc

At the 8th day of development, the blastocyst is partially embedded in the endometrial stroma. In the area over the embryoblast, trophoblast has differentiated into two layers;

• An inner layer of mononucleated cells, the cytotrophoblast, and
• An outer layer of multinucleated zone or cells without distinct cell boundries, the syncytotrophoblast. These cells invade the endometrium of uterus and supplies nutrition to the embryo.

Cells of inner cell mass or embryoblast also differentiate into two layers:

 

• A layer of small cuboidal cells adjacent to the blastocyst cavity is known as the hypoblast layer, and
• A layer of high columnar cells adjacent to the amniotic cavity is known as the epiblast layer.

 

Cells of each germ layer form a flat disc and together are known as "bilaminar germ disc".

Between epiblast and hypoblast, there is an extra-cellular basement membrane that separate the bilaminar germ disc.

At the same time, a small cavity appears above the epiblast. This cavity enlarges to become the amniotic cavity.

Those epiblast cells adjacent to the cytotrophoblast cells are called amnioblast, and together with the rest of the epiblast, line the amniotic cavity.

Fig: Cytotrophoblast and syncytotrophoblast cell

On the other hand, the hypoblast encircle the unilaminar blastocele forming bilaminar blstocele that gives rise to the primary yolk sac. 

Now the bilaminar germ disc is formed between the primary amniotic cavity above and the primary yolk sac below.

Trilaminar germ disc 

 (3^rd week of development)

Fig: Functional zones of ectoderm

The most characteristics events occurring during the 3^rd week is the Gastrulation, the process that establishes all three germ layers in the embryo, and these are as follows;

A. Ectoderm,

B. Mesoderm, &

C. Endoderm.

Appearance of germ layers 

At first stage, the germ disc is circular, then it becomes elongated, indicating the future axis of the embryo. 

One end becomes broad (cephalic end) and the other end becomes narrow (caudal end).

Three functional zones are differentiated in the ectodermal layer of the germ disc such as;

A. Surface ectoderm

B. Neural plate

C. Pluripotent cellular zone

    A. Surface ectoderm

The cells of cephalic end and the peripheral margin constitute the surface ectoderm, that gives rise to the epidermis of skin.

B. Neural plate

The cells on the depressed area at the central part of the surface ectoderm form an elongated plate, which gives rise to the future nervous system.

C. Pluripotent cellular zone

A group of fast proliferating cells are accommodated at the caudal part of the germ disc. 

They differentiated very rapidly and form a linear opacity in the midline, called primitive streak.

Gastrulation begins with the formation of primitive streak on the surface of the epiblast. 

Initially the cells of epiblast, i.e. pluripotent cells migrate towards the primitive streak.

On arrival in this region of the primitive streak, they become flask shape, detach from the epiblast, and slip beneath it. 

This inward movement is known as invagination and this invagination procedure between epiblast and hypoblast is known as gastrulation.

Fig: Gastrulation

Once the cells have invaginated, some of them displace the hypoblast, creating the embryonic endoderm, while some of them come to lie between the epiblast and newly created endoderm to form mesoderm. 

Cells remaining in the epiblast then form ectoderm. Thus the epiblast, through the process of gastrulation, is the source of all the germ layers and cells in these layers will give rise to all of the tissues and organs in the embryo.

Therefore all the three components of the tri-laminar germ disc, i.e. ectoderm, mesoderm and endoderm are derived from pluripotent epiblast by forming primitive streak.

Conclusion:

From the above statement, it is concluded that immediately after fertilization, the zygote undergoes a series of mitotic cell division results in the formation of morula.

The morula consists of centrally located inner cell mass that gives rise to the embryo  while the outer cell mass contribute to the formation of placenta.

The morula then enters the uterine cavity where fluid penetrate in the intercellular space forming a single cavity called blastocele and this stage is called blastocyst or blastula.

The blastocyst gradually loses the zona pellucida allowing implantation to begin which is initiated by trophoblast cells.

The trophoblast cells by secreting proteolytic enzymes dissolve zona pellucida and epithelium of uterine mucosa for attachment to establish nutritional supply to the embryo.

After implantation bilaminar germ disc is formed, initiated by inner cell mass/embryoblast which is further differentiated into epiblast and hypoblast.

The epiblast cells give rise to the future ectoderm as well as amniotic cavity above where as hypoblast cells give rise to the future endoderm as well as the yolk sac below.

With the formation of bilaminar  germ disc, the process of gastrulation begins  from ectoderm that gives rise to the trilaminar germ disc.

The ectoderm is further differentiated into three functional zones as surface ectoderm, neural plate and pleuripotent cellular zone.

The pleuripotent cellular zone gives rise to the primitive streak and with the formation of primitive streak, the process of gastrulation begins that gives rise to the mesoderm.

In this way, all the three components of the trilaminar germ disc, i.e. ectoderm, 

mesoderm, and endoderm are derived from pleuripotent epiblast by forming primitive streak.

Upon reading this post, I hope that you got to know about cleavage,  Implantation as well as the development of bilaminar and trilaminar germ disc in animal.


Thank you.

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