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Development of the Placenta

Jul 22, 2024

Development of the Placenta

Basics

  • Focus on the development and hormonal functions of the placenta.
  • Some clinical correlations included.
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Cleavage and Implantation

Cleavage Process

  1. Starts in the ampulla of the fallopian tubes. Involves the fusion of sperm and secondary oocyte (in metaphase 2), forming a zygote.
  2. Development Stages:
    • Zygote: Surrounded by zona pellucida.
    • 2-Cell Stage: Divides from zygote.
    • 4-Cell Stage: Further cleavage.
    • 8-Cell Stage: More division.
    • Morula: 16-cell stage, hollow ball with zona pellucida.
    • Blastocyst: Consists of outer cell mass (trophoblast) and inner cell mass (embryoblast). Develops from water entering the morula forming a cavity (blastocoel).

Implantation

  • Blastocyst: Implants into the endometrium, interacting with different layers (decidua).
  • Endometrial Layers:
    • Decidua basalis: Basement part.
    • Decidua functionalis: Functional layer.
    • Decidua capsularis: Covers the trophoblast not interacting with the uterine lining.

Differentiation and Invasion

Trophoblast Differentiation

  • Cytotrophoblast: Single cell layer.
  • Syncytiotrophoblast: Multinucleated cells, secretes hydrolytic enzymes aiding invasion into the endometrium.

Functions

  • HCG secretion: Maintains progesterone production preventing menstruation.

Embryoblast Differentiation

  • Bilaminar Disk Formation: Epiblast and hypoblast.
  • Further development into ectoderm, endoderm, mesoderm.

Chorionic Cavity and Villi Formation

  • Formation of extraembryonic mesoderm.
  • Chorionic villi development: From primary (cytotrophoblast) to secondary (extra-embryonic mesoderm) and tertiary villi (with fetal blood vessels).
  • Umbilical cord: Contains two umbilical arteries, one umbilical vein.
  • Formation of chorionic frondosum: Interaction with decidua basalis forming the placenta.

Development Timeline

  • Primary villus formation: Days 13-14.
  • Secondary and tertiary villi formation: Weeks 3-4.
  • Cotyledons formation: Fourth to fifth month.
  • Loss of cytotrophoblastic layer: Post-20 weeks to improve maternal-fetal exchange efficiency.

Placental Functions

Metabolic Functions

  1. Gas Exchange: Oxygen and CO2.
  2. Nutrient Delivery: Glucose (facilitated diffusion/active transport), amino acids, fatty acids, water-soluble vitamins (B vitamins), IgG antibodies (passive immunity).
  3. Waste Removal: Urea, uric acid.

Hormonal Functions

  1. Estrogen & Progesterone: Maintain uterine lining, support fetus development.
  2. Thyroid Hormone: CNS development.
  3. Human Placental Lactogen: Supports glucose/fatty acid supply for fetus, reduces maternal insulin sensitivity.
  4. Relaxin: Promotes ligament laxity in pubic symphysis for childbirth.
  5. Corticotropin-Releasing Hormone: Stimulates cortisol for fetal lung development and surfactant production.

Clinical Correlations

  • Placenta accreta/increta/percreta: Abnormal placental attachment.
  • RH incompatibility: RhoGAM to prevent hemolytic disease of the newborn.
  • TORCH Infections: Toxoplasmosis, Others (HIV, syphilis, HBV), Rubella, Cytomegalovirus, Herpes Simplex.
  • Zika Virus: Potential for vertical transmission.

Conclusion

  • Extensive overview of placental development and its important functions in fetal nourishment and protection.
  • Emphasize on fetal-maternal exchange and hormonal balance ensuring fetal development.

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Full transcript