Human Embryology: Implantation of the Blastocyst

Study Notes

Implantation begins 2-3 days after the blastocyst enters the uterine lumen and consists of three stages: apposition, adhesion, and penetration.
Apposition: The polar trophoblast comes into contact with the endometrium, and the endometrium increases mitosis and produces pinopods, which withdraw fluid from the lumen by pinocytosis.
Adhesion: The microvilli disappear, and the production of 'sticky' glycoproteins leads to contact over a large surface area.
Penetration: The trophoblast migrates between endometrial cells, and a syncytiotrophoblast forms, synthesizing trophoblast-specific proteins (e.g., SP1 and hCG).

Decidual cells form in the endometrial stroma, rich in glycogen and lipid.
The trophoblast forms outer syncytial and inner cellular layers, and the embryonic disc forms ectodermal and endodermal layers.
The endoderm gives rise to cells that migrate onto the inner layer of the mural trophoblast, forming Hauser's membrane.
The amniotic cavity forms between the cytotrophoblast and the epithelial layer of the embryonic disc.
At 11 days, Hauser's membrane acquires an intermediate layer of mesoderm, and the blastocoelic cavity is now called the primary yolk sac.
The secondary yolk sac forms by 'collapse' of the endodermal lining of the primary yolk sac.
The 'extra-embryonic coelom' forms in the site of the primary yolk sac and surrounds the yolk sac, embryonic disc, and amnion.
A layer of mesoderm separates the endoderm of the extra-embryonic coelom from the underlying structures.
The mesoderm between the amnion and trophoblast is called the embryonic stalk, which will form the umbilical cord.

At the time of the first missed period, the embryonic disc is still bilaminar, but a node of epithelial cells, the primitive streak, is forming at the caudal end.
The placenta and membranes have undergone extensive development by this stage.
Lacunae form in the polar syncytiotrophoblast, which develop a brush border and fuse with maternal capillaries, yielding a blood-filled space that will become the intervillous space.
At 13 days, primary villous stems are formed with a lining of syncytiotrophoblast and a core of cytotrophoblast and mesoderm.
The cytotrophoblast grows as 'columns' through the syncytiotrophoblast to make contact with maternal decidua, and then forms a 'shell' enclosing the lacunae and syncytiotrophoblast.
The shell is penetrated by maternal spiral arteries and venules.
During the early second trimester, a second wave of trophoblast invasion takes place from the trophoblast wall.
During the 3rd week of embryonic life, the villous stems become vascularized and establish continuity with other vessels developing in the body stalk.