Asked Dec 5, 2019

Can I get clariication on the week 2 to 3 events of development (bilaminar disk; amniotic cavity, yolk sac, chorion: trilaminar disk; gatrulation, notochord, primitive heart)?


Expert Answer

Step 1

Embryogenesis is the process that refers to the development of the fetus during the first eight weeks after fertilization. The circulatory, excretory, and neurologic systems all begin to develop during this stage.

Step 2

Week 2 of embryonic development encompasses the implantation process and blastocyst differentiation. The blastocyst differentiates into extraembryonic and embryonic tissues and proliferate. It then embeds itself in the uterine lining via the trophoblast cells. Thus begins the process of implantation, which signals the end of the pre-embryonic stage of development. In placental animals, this is the first interaction between the conceptus ( embryonic tissues; zygote) and the maternal uterine wall. The implanting embryonic tissues secrete a hormone i.e human chorionic gonadotropin that initiates maternal hormonal changes and stops the menstrual cycle.

When implantation succeeds and the blastocyst adheres to the endometrium, the superficial cells of the trophoblast fuse with each other, forming the syncytiotrophoblast, a multinucleated body that digests endometrial cells to firmly secure the blastocyst to the uterine wall. 

Step 3

 During the third week of development, the two-layered disc of cells becomes a three-layered disc through the process of gastrulation, during which the cells show a transition from totipotency to multipotency. The embryo forms an indentation called the primitive streak along the dorsal surface of the epiblast. A node at the caudal or “tail” end of the primitive streak emits growth factors that direct cells to multiply and m...

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Embryology and Organogenesis

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