Human Embryology: Implantation of the Blastocyst

Implantation of the Blastocyst

• 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).

Early Development of the Placenta and Embryo

• 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.

Development of the Embryo and Placenta

• 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.