Embryonic development - week 1 and 2
The first through eighth weeks after fertilization are called the embryonic. Week 1 • Within a day, the zygote begins mitotic cell division (cleavage) forming blastomeres. By the 4th day, the blastomeres have formed a solid ball called a morula. • The morula enters uterine cavity around day 4 or 5. • Glycogen-rich secretions from the endometrial glands nourish the developing morula. • By day 5, the embryo has shed the zona pellucida and become a hollow ball of cells known as the blastocyst. • Within the blastocyst, new cells move to form the inner cell mass and trophoblast cells. • By day six, the blastocyst implants into the endometrium. Week 2 • During the second week after fertilization, the implanted blastocyst stage continues to develop. • On the eighth day, the trophoblast cells of the blastocyst differentiate into two layers: • The inner cytotrophoblast layer. • The outermost syncytiotrophoblast layer secretes enzymes that promote further implantation as well as human chorionic gonadotropin (hCG), a hormone that acts to maintain the endometrium. • Also on the eighth day, the cells of the inner cell mass differentiate into two layers: • The hypoblast layer. • The epiblast layer. • Both layers together form the bilaminar embryonic disc. • During the ninth day, the epiblast cells begin to multiply and migrate, forming the fluid-filled amniotic cavity. • At the same time, the multiplying hypoblast cells begin to develop into a yolk sac. • As a result, the bilaminar embryonic disc is positioned between the amnion and yolk sac. • By the twelfth day, differentiation of yolk sac cells forms an extraembryonic mesoderm. • This new layer, along with the syncytiotrophoblast and cytotrophoblast layers, form the chorion. • The chorion grows to form the connecting stalk, which is the future umbilicus. • The chorion, destined to form the embryonic part of the placenta, is important in the exchange of nutrients between maternal blood and the embryo.
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