4 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
6 Lacunar spaces appear in cyncitiotrophoblast. 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
8 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
10 Formation of extra-embryonic coelom and connecting stalk. 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
11 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
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