General embryology: Summary of different event of 1st, 2nd and 3rd week of development

1^st week of development:

The main events

1.       Fertilization  

2.        Zygote formation

3.        Initiation of cleavage division

4.       Formation of morula

5.     Morula convert into blastocyst  

6.     Implantation

2^nd week of development:

It is also known as the week of twos:

1. the trophoblast (which forms supporting structure of embryo) differentiate into two layers 

1. the cytotrophoblast    
2.  the syncytotrophoblast

2.  the embryoblast forms two layers

i) epiblast and

 ii) hypoblast

3. the extraembryonic mesoderm splits into two layers                                             i)somatopleuric mesoderm and                                                                   ii)splanchnopleuric mesoderm
4. two cavities form

 i) amniotic and

 ii) yolk sac

5. Other features:

• uteroplacental circulation begin

• primary villi of placenta is formed

• chorionic cavity is formed

3^rd week of development:

The main events-

1. Appearance of  primitive streak: it is the thickening of the  epiblast at the caudal end of the embryonic disc
2. Gastrulation : formation of three germ layers – ectoderm, mesoderm and endoderm
3. Formation of the notochord:
4. growth of embryonic disc
5. establishment of body axes
6. tertiary villi are formed by the end of the third week