Implantation

Blastocyst Formation - Getting Ready to Stick

• Day 4: Morula enters the uterine cavity.
• Uterine fluid penetrates the zona pellucida, forming the blastocoel (cavity).
• This reorganizes the morula into a blastocyst, composed of:
  • Trophectoderm: Outer cell layer; forms the placenta.
  • Embryoblast (Inner Cell Mass): Becomes the embryo.
• Day 5-6: "Hatching" occurs. The blastocyst sheds the zona pellucida, a necessary step for implantation. Trophoblastic proteases degrade the zona.

⭐ Initial adhesion of the blastocyst to the uterine wall is mediated by L-selectin on trophoblast cells binding to carbohydrate receptors on the uterine epithelium.

The Implantation Window - Perfect Timing

• A short receptive period in the mid-luteal phase, days 20-24 of a 28-day cycle (6-10 days post-ovulation).
• Driven by ↑ progesterone from the corpus luteum, priming the estrogen-prepared endometrium.
  • Promotes secretion of glycogen-rich fluid.
  • Induces stromal cell decidualization.
• Adhesion molecules are key for blastocyst capture:
  • Trophoblast L-selectin binds endometrial carbohydrate ligands.
  • Integrins (e.g., $α_vβ_3$) ensure stable attachment.

⭐ Pinopodes, progesterone-dependent endometrial protrusions, absorb uterine fluid, bringing the blastocyst closer to the endometrium for attachment.

Invasion & Decidualization - The Great Attachment

• Syncytiotrophoblast Invasion:

  • This aggressive, multinucleated outer layer secretes proteolytic enzymes (e.g., matrix metalloproteinases) to digest the endometrial stroma.
  • It rapidly expands, forming lacunae that anastomose with maternal sinusoids, establishing primordial uteroplacental circulation.
  • Secretes human chorionic gonadotropin (hCG).

• Endometrial Decidualization:

  • Under progesterone's influence, endometrial stromal cells swell, accumulating glycogen and lipids to become decidual cells.
  • This reaction creates a nutrient-rich, immunologically privileged site for the conceptus.
  • Forms the maternal component of the placenta (decidua basalis).

⭐ The decidual reaction is crucial for preventing excessive trophoblastic invasion into the myometrium. A defective decidual layer can lead to placenta accreta, increta, or percreta, causing severe postpartum hemorrhage.

Clinical Correlates - When Things Go Wrong

• Ectopic Pregnancy: Implantation outside the uterine cavity.
  • Most common site: Ampulla of the fallopian tube.
  • Presents as a surgical emergency (pain, bleeding).
• Placenta Previa: Implantation over or near the internal cervical os.
  • Key symptom: painless third-trimester bleeding.
• Placenta Accreta Spectrum: Abnormal placental invasion into the uterine wall.
  • Risk ↑ with prior C-sections, uterine surgery.

⭐ A history of cesarean delivery significantly increases the risk for both placenta previa and accreta.

• Implantation occurs ~6-12 days post-fertilization, most commonly on the posterior uterine wall.
• The blastocyst must hatch from the zona pellucida before embedding.
• Syncytiotrophoblast invades the endometrium and secretes hCG to maintain the corpus luteum.
• The endometrium transforms via the decidual reaction to support the pregnancy.
• Ectopic pregnancy most commonly occurs in the ampulla of the fallopian tube.
• Placenta previa is implantation over or near the internal cervical os.