Formation of morula

Dr. Sherif Fahmy

Blastocyst (at 7th day)

Dr. Sherif Fahmy

8th day of pregnancy

Dr. Sherif Fahmy

Second Week of Pregnany

8th day: Blastocyt is partially implanted to the

endometrium which is called decidua. Trophoblastic cells proliferate rapidly to be

differentiated into 2 layers: cytotrophoblast (inner cellular layer) and outer syncytiotrophoblast (sheet of protoplasm).

Formation of amniotic cavity between epiblast and cytotrophoblast.

Cells of inner cell mass (embryoblast) become arranged into 2 layers: Hypoblast (primary endoderm) and epiblast (primary ectoderm), so forming a bilaminar germ disc. Dr. Sherif Fahmy

9th & 10th days

Dr. Sherif Fahmy

9th day:

Implantation is completed and the penetration defect in endometrium is closed by fibrin clot.

Formation of primitive yolk sac (exocoelomic cavity). This occurs by proliferation of cells from hypoblast which descend to line the trophoblast at the original blastocele called Heuser’s membrane.

10th day:- Lacunar spaces appear in

cyncitiotrophoblast. Dr. Sherif Fahmy

11th & 12th days

Dr. Sherif Fahmy

11th & 12th days: Implantation defect is covered by surface

epithelium. Blastocyst produces a slight protrusion into

uterine cavity. Lacunar spaces become filled with maternal

blood due to deeper penetration of cyncytiotrophoblast to the endometrium.

Formation of extraembryonic mesoderm in the inner aspect of cytotrophoblast. Now trophoblasts is called chorion and blastocyst is called chorionic vesicle.

Large spaces appear in the mesoderm which communicate to form extra-embryonic coelom except at part connected to caudal end of embryo called connecting stalk. Dr. Sherif Fahmy

13th day

Dr. Sherif Fahmy

13th day: Formation of secondary yolk sac which

becomes lined with endodermal cells and reduced in size due to punching off a part from it.

Formation of extra-embryonic coelom and connecting stalk.

Formation of primary chorionic villi. Development of blood vessels in

connecting stalk. Formation of prochordal plate in which

the ectoderm and endoderm are fused together. Dr. Sherif Fahmy

Summary of the Second Week of Pregnancy

Through this week, the following structures are formed: Two cavities: amniotic and yolk. Two layers of embryonic disc: ectoderm and

endoderm. Two layers of trophoblast: inner

cytotrophoblast and outer cyncytiotrophoblast.

Dr. Sherif Fahmy