Gastrulation

*Development of trophoblast

-When the blastula implants to the uterine lining, trophoblast starts proliferating outside the cell while the cell membrane will disintegrate 

-Thus, cytoplasm will form which consists of the nuclei of the cells 

-After that, cytoplasm start to fuse with one another, forming syncytiotrophoblast

-Cells with well defined membrane and have nuclei inside of them will be called cytotrophoblast

-Syncytophoblast then releases a kind of hydrologic enzyme which helps them merge with maternal blood vessels in the uterine lining. Thus, embryo can receive oxygen, nutrients, hormones, etc to grow (placenta)

-Around day 28, syncytiotrophoblast also produces a hormone called human chorionic gonadotropin which stimulates the corpus luteum of the ovary to keep producing progesterone. As a result, vessels inside the uterine lining will not spasm and rupture. Therefore, embryo will not shed

*Gastrulation and development of embryoblast

-Embryoblast will keep differentiating and the new structure will be called bilaminar disc:

+The top layer is known as the epiblast while the bottom layer is called hypoblast. Between these two layers, a thickening layer called prechordal plate is formed. 

+The region above epiblast is filled with a clear, slightly yellowish liquid called amniotic fluid (protects fetus from injury and temperature change) while the region below hypoblast is called yolk sac (contribute to the vascular system of the developing embryo)

-From week 2 to week 3, epiblast will form a thick area called primitive streak and primitive node which is located at the top of primitive streak. 

-Some of the cells near the edge of the primitive streak starts secreting a certain type of chemical called fibroblast growth factor 8 which moves to near epiblast cells and triggers the intercellular process. Consequently, a special protein is produced to inactivate E. Coherent protein (this protein helps 2 adjacent epiblast cell sticking with each other) 

-Hence, 2 adjacent epiblast cells will move apart (epithelial migration) 

-At this point, near primitive streak and primitive node, there are new structures called primitive groove and primitive pit because of the space between epiblast cells

-These cells start moving through the primitive groove and down to the hypoblast to replace this structure. Thus, a new layer called endoderm is formed

-Some of the epiblast cells also keep moving down, laterally, and forward to form a second new layer called mesoderm

-At this moment, epiblast will be known as the ectoderm layer

-The formation of ectoderm, endoderm, and mesoderm is known as gastrulation

-Epiblast cells also move through primitive pit and towards the prechordal plate to make a tube called notochord (has an important role in vertebrate development since notochord serves as a midline tissue that provides directional signals to surrounding tissue during development,) which is located underneath the ectoderm and above the endoderm