11. Post fertilization - embryogenesis - cleavage and implantation

The total time required for the development of the baby until its delivery from a zygote is \(280\) \(days\). The period is called gestation period or pregnancy.

 

The process of embryogenesis takes place in four stages,  which include: 

 

1. Cleavage and formation of blastula

 

2. Implantation

 

3. Gastrulation

 

4. Organogenesis

Cleavage, thus converts the single-celledzygote to a multicellularstructure. The first cleavage takes place about \(30\) \(hours\) after fertilization, where the zygote divides into two cells - blastomeres. The next division take place within forty \(hours\) after fertilization.

 

While the third division takes place for nearly three days after fertilization. During cleavage, the embryoslowly moves down the fallopian tube towards the uterus. At the end of the fourth day, embryo reaches the uterus where it has \(8\) - \(16\) blastomeres, called morula.

 

A gif depicting the developmental stages of an embryo:

 

 

At the next stage, the morula rapidly divides and a multicellular mass that comprises \(64\) cells blastomeres are produced.

 

The blastomeres rearrange into an outer layer of smaller cells called trophoblast and an inner group of larger cells called inner cell mass. There is also a hollow cavity called blastocyst cavity or blastocoel. The structure is called as blastocyst.  

 

After reaching the uterus, the blastocyst is attached and embedded to endometrium of the uterus.

The blastocyst gets embedded in a pit that is formed in the endometrium. The inner cell mass of blastocyst gives rise to the embryo. The cells of the trophoblast forms the parts of the placenta.

 

The fertilized egg becomes implanted in about \(6\) to \(7\) days after fertilization.