Embryonic Development Series: Part 2 - Implantation

Review of First Week Development

• Conceptus is moved from site of fertilization to site of implantation.
• Morphological changes: Zygote → Two-cell stage → Four-cell stage → Morula → Blastocyst.
• Blastocyst consists of:
  • Inner cell mass (Embryoblast)
  • Outer cell mass (Trophoblast)
• Blastocyst cavity and embryonic pole play roles in the hatching and implantation processes.

Second Week of Development

• Implantation marks the establishment of pregnancy.
• Other second-week processes (to be covered in later lectures):
  • Morphological changes to cell masses.
  • Fetal membrane establishment.
• The second week is known as the "week of twos."

Objectives of the Lecture

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

Implantation Defined

• Implantation: Blastocyst attaches and embeds into the endometrial lining of the uterine wall.
• Normal Site: Upper part (fundus) of the uterus.
• Process involves invasion by the trophoblast.

Uterine Anatomy

• Uterus parts: Fundus, body, cervix.
• Uterine layers: Perimetrium, Myometrium, Endometrium.
• Endometrium has Stratum Basalis and Stratum Functionalis.
• Normal implantation involves borrowing into the Stratum Functionalis.

Key Steps in Implantation

1. Hatching: Blastocyst extrudes from zona pellucida.
2. Attachment: Blastocyst attaches to endometrial lining using embryonic pole.
3. Trophoblast Differentiation: Into syncytiotrophoblast and cytotrophoblast.
4. Invasion and Embedding: Syncytiotrophoblast burrows into endometrium.
5. Primitive Uteroplacental Unit: Formation of trophoblastic lacunae and maternal sinusoids.
6. Repair: Endometrial defect repaired by fibrin clots and regenerating epithelium.

Hormonal Changes During Implantation

• Pre-Implantation: Estrogen and progesterone levels rise and fall, leading to menstruation if no pregnancy.
• During Implantation: hCG from trophoblast maintains corpus luteum, sustaining progesterone and estrogen levels, preventing menstruation.
• Post-Corpus Luteum: Placenta takes over hormone production after 2-3 months.

Disorders of Implantation

1. Ectopic Pregnancy: Implantation outside endometrial cavity.
   • Commonly tubal pregnancy leading to tubal rupture and embryonic demise.
2. Placenta Previa: Placenta forms in lower uterine segment, can cause bleeding and fetal risk.
3. Abnormal Depth of Implantation:
   • Placenta accreta, increta, percreta - varying severity of uterine wall invasion.
4. Early Pregnancy Loss: Miscarriage or spontaneous abortion before 20 weeks.
   • Causes: Genetic anomalies, hormonal imbalances, infections, etc.
   • Symptoms: Vaginal bleeding, lower abdominal pain.
   • Terms: Anembryonic pregnancy, embryonic demise.

Conclusion

• Disorders of implantation can lead to serious complications for both mother and fetus.
• Next lecture will cover other events of the second week of development excluding implantation.