Implantation of the Blastocyst

Timeline of Implantation

• 6 to 8 days after fertilization: Implantation begins.
• End of Day 8: Blastocyst is embedded in the endometrium.

Components of the Blastocyst

• Outer Cell Mass: Trophoblast
• Inner Cell Mass: Embryoblast

Differentiation of the Trophoblast

• Contact with Endometrium: Differentiates into:
  • Cytotrophoblast: Inner layer
  • Syncytiotrophoblast: Outer layer

Differentiation of the Embryoblast

• Forms a bilaminar embryonic disc:
  • Hypoblast: Lower layer
  • Epiblast: Upper layer

Formation of Cavities

• Amniotic Cavity: Appears between epiblast and cytotrophoblast.
• Exocoelomic Membrane: Formed by migrating hypoblast cells, covers inner cytotrophoblast.
• Primitive Yolk Sac: Formed by exocoelomic membrane and hypoblast cells.

Development by Day 9

• Blastocyst fully embedded in uterine wall.
• Growth:
  • Cytotrophoblast rapidly growing.
  • Syncytiotrophoblast expanding with lacunae forming.

By Day 12

• Lacunae: Fuse to form lacunar networks.
• Capillaries: Dilate to form maternal sinusoids.
• Uteroplacental Circulation: Established as enzymes erode sinusoids, allowing maternal blood flow.

Appearance of Extraembryonic Mesoderm

• Location: Between cytotrophoblast and primitive yolk sac.
• Cavity Formation: Large cavities fuse to form the chorionic cavity.

By Day 13

• Secondary Yolk Sac: Formed as exocoelomic cavity is pinched off.

By End of Second Week

• Chorionic Cavity: Enlarges.
• Connecting Stalk: Joins bilaminar embryonic disc to trophoblast, precursor to the umbilical cord.