first and second week of development of embrylogy

morula stage, The Blastulation Stage, second week stage

1. Embryology laboratory Dr. Sabrin Sami

2. Morula stage

3. Within 24 hours after fertilization, the zygote initiates a rapid series of mitotic cell divisions called cleavage . the large zygote subdivided into many smaller daughter cells called blastomeres . When the blastomeres number reaches to the 16-32 cells, the embryo called a morula. The centrally placed blastomeres are now called the inner cell mass , the inner cell mass gives rise to the embryo proper called the embryoblast .

4. The outside blastomeres are called the outer cell mass constitute the trophoblast which supplies Nutrition to the embryo Coverings of the embryo. Acts as a primary source of the fetal component of the placenta.

5. The Blastulation Stage

6. Between the 4 th and 5 th day after the fertilization, the morula reaches the uterine cavity. Due to accumulation of fluid inside the morula the embryoblast separated from the trophoblast . As a result the blastocyst cavity ( blastocoel ) forms.

7. The embryo is now called a blastocyst . The embryoblast cells form a compact mass at one side of this cavity, The trophoblast organizes into thin, single- outer layere . This stage is called the blastulation .

8. Implantation of blastocyst The implantation occurs at the 6 th day of the first week. The blastocyst increased in size and hatches out of the zona pellucida. The trophoblast of the blastocyst can interact directly with the endometrium and gets implanted on the wall of the uterus.

9. The embryoblast begin to differentiate into two layers: An upper layer of columnar cells, called the epiblast layer ( primitive ectoderm ). A lower layer of small cuboidal cells adjacent to the blastocyst cavity, called the hypoblast layer ( primitive endoderm ). The resulting two-layered embryoblast is called the bilaminar embryonic disc .

10. Fully implanted Second week of development

11. As the embryo begins to implants, the trophoblast begins to differentiate into two layers : An outer layer called the syncytiotrophoblast , form the syncytium . An inner layer called the cytotrophoblast . Trophoblast facilitates implantation by secreting proteolytic enzymes to erode the uterine endometrium.

12. By day 9 , The blastocyst is more deeply embedded in the endometrium, and the penetration defect in the surface epithelium is closed by a fibrin coagulum . The vacuoles appear in the syncytium, fuse together and form large lacunae syncytial lacunae , is known as the lacunar stage . These lacunae fused to each other to form large lacunae that becomes continuous with the maternal blood sinusoids.

13. These represent the primary uteroplacental circulation. By the 11 th to the 12 th day of development, the blastocyst is completely embedded in the endometrial stroma. At day 8 , fluid begins to collect between the epiblast and trophoblast Amniotic cavity appears Cells from the epiblast line this cavity and called amnioblasts cells.