Development of the Placenta

Overview

• Discussed the development and hormonal functions of the placenta
• Introduced key stages and structures involved in placental development
• Mentioned clinical correlations such as placenta accreta, increta, and percreta

Pre-Development Context

• Started from the ampulla of the fallopian tubes
• Sperm and secondary oocyte fuse to form the zygote
• The zygote undergoes cleavage stages:
  • 2-cell stage
  • 4-cell stage
  • 8-cell stage
  • 16-cell stage (morula)
• Formation of the blastocyst from the morula
  • Inner cell mass (embryoblast)
  • Outer cell mass (trophoblast)

Implantation

• Blastocyst attaches to the uterine lining (endometrium)
  • Uterine layers: endometrium, myometrium, perimetrium
• Process involves loose and tight attachment mechanisms:
  • Loose: Microvilli of trophoblast and pinopods of endometrium
  • Tight: Integrins (trophoblast) and selectins/collagen (endometrium)
• Chemokine release stabilizes attachment
• Trophoblast differentiates into:
  • Cytotrophoblast (single layer)
  • Syncytiotrophoblast (multinucleated structure)

Early Placental Development

• Syncytiotrophoblast releases enzymes for stromal invasion
• Secretes hCG to sustain corpus luteum and high progesterone levels
• Embryoblast forms bilaminar disk (epiblast and hypoblast)
• Formation of amniotic cavity and primary yolk sac

Lacunar and Intervillous Spaces Formation

• Around day 9: formation of lacunae (spaces in syncytiotrophoblast)
• Around day 12: lacunae fill with maternal blood, become intervillous spaces
• Extraembryonic mesoderm formation around amniotic cavity and primary yolk sac

Chorionic Development

• Day 13-14: Formation of extraembryonic coelom (chorionic cavity)
• Cytotrophoblast forms primary villi, penetrates syncytiotrophoblast
• Development of outer cytotrophoblastic shell
• Week 3-8: Transition to secondary and tertiary villi
  • Secondary villi include extraembryonic mesoderm
  • Tertiary villi have developing fetal blood vessels
• Formation of umbilical cord with two umbilical arteries and one umbilical vein

Maternal-Fetal Exchange

• Placental exchange from end of week 3 to week 20
  • Maternal blood in intervillous spaces exchanges gases, nutrients, and wastes with fetal blood
• Around week 20: cytotrophoblast layer regresses for efficient exchange
• Formation of 15-20 cotyledons (placental subdivisions) by decidual septa around month 4-5

Hormonal Functions

• Placenta produces several hormones:
  • Estrogen and progesterone (sustain endometrium, thicken lining, fetal development)
  • hCG (maintains corpus luteum, progesterone levels)
  • Human placental lactogen (alters maternal insulin sensitivity, increases glucose and fatty acids for fetus)
  • Relaxin (relaxes ligaments, widens pelvic inlet/outlet)
  • Corticotropin-releasing hormone (CRH) (promotes fetal cortisol production for lung development)

Clinical Correlations

• Risks of improper placental attachment:
  • Placenta accreta: penetrates decidua basalis
  • Placenta increta: penetrates myometrium
  • Placenta percreta: penetrates perimetrium
• Importance of passive immunity via IgG antibodies
• Risks of transplacental infections (TORCH infections: Toxoplasmosis, Others [HIV, syphilis, hepatitis], Rubella, CMV, Herpes)

Summary

• Placenta develops from zygote stage, differentiates through various stages, attaches to the uterine lining
• Forms complex structures for maternal-fetal exchange of gases, nutrients, and hormones
• Produces crucial hormones for pregnancy maintenance and fetal development
• Has significant clinical implications in maternal and fetal health