Lecture: Implantation and Disorders of Implantation

Overview

• Focus on the process of implantation and disorders associated with it.
• Review of the first week's development: Fertilization to Blastocyst.
• Introduction to the second week where implantation occurs, establishing pregnancy.

Review of First Week

• Stages of Development:
  • Zygote → Two-cell stage → Four-cell stage → Morula → Blastocyst.
• Blastocyst Structure:
  • Inner cell mass (Embryoblast)
  • Outer cell mass (Trophoblast)
  • Blastocyst cavity and embryonic pole.

Second Week of Development

• Known as the "Week of Twos."
• Focus on implantation.
• Other processes like morphological changes and fetal membranes will be discussed in future lectures.

Objectives of Lecture

1. Define implantation and its normal site.
2. Key steps during implantation.
3. Hormonal changes during implantation.
4. Conditions due to abnormal implantation.

Implantation

• Definition: The process by which the blastocyst attaches and embeds into the endometrial lining of the uterine wall.
• Normal Site: Upper part of the uterus (fundus).
• Process:
  • Invasion and embedding by the trophoblast.

Anatomy of the Uterus

• Layers:
  • Perimetrium (outermost)
  • Myometrium (muscular, with blood vessels)
  • Endometrium (innermost, with stratum basales and functionalists)
• Implantation Extent: Up to the junction between stratum functionalists and stratum basales.

Key Steps in Implantation

1. Hatching:
   • Blastocyst hatches from the zona pellucida around day 5-6.
2. Attachment:
   • Blastocyst attaches to the endometrial lining using the embryonic pole.
3. Trophoblast Differentiation:
   • Into syncytiotrophoblast (outer) and cytotrophoblast (inner).
   • Syncytiotrophoblast invades endometrium.
4. Formation of Trophoblastic Lacunae:
   • Communicate with maternal sinusoids to form primitive uteroplacental unit.
5. Repair of Endometrial Defect:
   • By fibrin clots and epithelial regeneration.

Hormonal Changes

• Role of hCG:
  • Produced by syncytiotrophoblast to maintain corpus luteum.
• Corpus Luteum:
  • Produces progesterone to sustain pregnancy until placenta forms.
• Menstruation Suppression:
  • Due to sustained progesterone levels.

Disorders of Implantation

1. Ectopic Pregnancy:
   • Implantation outside the endometrial cavity (common in fallopian tubes).
   • Risk of tubal rupture and embryonic demise.
2. Placenta Previa:
   • Low-lying placenta, risk of antepartum hemorrhage.
3. Abnormal Placentation:
   • Placenta accreta, increta, percreta, leading to retained placenta and postpartum hemorrhage.
4. Early Pregnancy Loss:
   • Miscarriage or spontaneous abortion before 20 weeks.
   • Risk factors include chromosomal abnormalities, hormonal disorders, infections, etc.

Conclusion

• Disorders highlight the complexity and potential complications of implantation.
• Next lecture will cover additional events of the second week of development.