Implantation of the blastocyst usually occurs 6-8 days after fertilization.
By day 8, the blastocyst has burrowed into the uterine wall, being completely embedded by day 9.
Day 8
The blastocyst is composed of two main components: the outer cell mass, the trophoblast; and the inner cell mass, the embryoblast.
As the trophoblast makes contact with the endometrium, it differentiates into two layers: an inner, cytotrophoblast; and an outer, syncytiotrophoblast.
The embryoblast differentiates into a bilaminar embryonic disc composed of two cell layers, the hypoblast, and the epiblast. Soon after the embryonic disc has formed, a cavity begins to appear between the epiblast and cytotrophoblast, known as the amniotic cavity.
Cells originating from the hypoblast begin to migrate, forming a thin membrane, which lines the inner surface of the cytotrophoblast. This is called the exocoelomic membrane. The exocoelomic membrane and cells of the hypoblast together form the walls of the primitive yolk sac.
Day 9
Once the blastocyst is completely embedded in the uterus wall, a plug called a fibrin coagulum forms in the gap created in the epithelium of the uterus by the blastocyst. At this time, the growth of the cytotrophoblast and syncytiotrophoblast is much faster than that of the bilaminar embryonic disc
Small holes called lacunae begin to form in the syncytiotrophoblast as it continues to expand.
Day 12
Capillaries in the endometrium surrounding the developing embryo dilate, forming maternal sinusoids. As the lacunae continue to expand, enzymes within the syncytiotrophoblast begin to erode the lining of the sinusoids and uterine glands. This allows anastomosis between the maternal sinusoids and lacunar networks to begin establishment of an uteroplacental circulation.
By day 12, the lacunae stop growing and fuse to form large interconnecting spaces, called lacunar networks.
A new population of cells appears between the inner surface of the cytotrophoblast and the outer surface of the primitive yolk sac, known as the extraembryonic mesoderm.
Large cavities begin to appear in the extraembryonic mesoderm. These gradually fuse to form one single cavity, called an extraembryonic coelom.
Day 13
Approximately 13 days after fertilization, a large portion of the exocoelomic cavity is pinched off, forming a smaller cavity, called the secondary yolk sac. A remnant also forms within the exocoelomic cavity, known as an exocoelomic cyst, which is eventually eliminated.
By the end of the second week of development, the chorionic cavity enlarges, and the bilaminar embryonic disc is joined to the trophoblast by a band of extraembryonic mesoderm called the connecting stalk.
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