- Overview
This lecture covers the detailed development of the placenta, its structural layers, hormonal functions, key exchange processes, and clinical correlations relevant to pregnancy.
Early Placenta Development
- Fertilization produces a zygote, which divides into morula and then blastocyst stages.
- The blastocyst has outer trophoblast (forms placenta) and inner cell mass (forms fetus).
- Implantation involves loose (microvilli and pinopods) then tight (integrins/selectins) attachment to the endometrium.
Trophoblast Differentiation and Implantation
- Trophoblast differentiates into cytotrophoblast (inner layer) and syncytiotrophoblast (outer, multinucleated layer).
- Syncytiotrophoblast invades endometrium and releases HCG to maintain progesterone, preventing endometrial shedding.
- Lacunae form within syncytiotrophoblast, becoming intervillous spaces filled with maternal blood.
Formation of Extraembryonic Structures
- Extraembryonic mesoderm forms between trophoblast and embryonic structures.
- Cavitation in extraembryonic mesoderm forms the chorionic cavity (extraembryonic coelom).
- Chorion and primary/secondary/tertiary villi form the framework for placental exchange.
Chorionic Villi and Vascularization
- Primary villi: projections of cytotrophoblast.
- Secondary villi: invaded by extraembryonic mesoderm.
- Tertiary villi: mesoderm differentiates into fetal blood vessels.
- Umbilical cord contains two arteries and one vein; fetal blood flow passes through chorionic and cotyledon arteries.
Placental Architecture and Exchange
- From week 4–20: placental layers include syncytiotrophoblast, cytotrophoblast, and fetal capillaries.
- After week 20, cytotrophoblast regresses for more efficient exchange.
- Branching of villi increases surface area for maternal-fetal exchange.
Maternal-Fetal Interface & Cotyledons
- By 4th–5th month, septa from decidua divide villi into cotyledons (15–20 per placenta).
- The placenta consists of fetal (chorionic frondosum) and maternal (decidua basalis) components.
Placental Membranes and Uterine Coverings
- Chorion, amnion, and decidua (basalis, capsularis, parietalis) line the uterus; all participate in placental structure.
- Chorionic frondosum participates in exchange, chorion laeve is less active.
Placental Functions
- Oxygen and nutrient transfer from mother to fetus; CO₂ and waste removal from fetus to mother.
- Transfers IgG antibodies, providing passive immunity.
- Acts as a barrier, but some pathogens (TORCH, Zika, Listeria) can cross.
Hormonal Functions of the Placenta
- Produces HCG to maintain corpus luteum and progesterone early in pregnancy.
- Produces estrogen and progesterone after 10–12 weeks to sustain endometrial lining.
- Human placental lactogen (hPL) modifies maternal metabolism for fetal benefit.
- Synthesizes relaxin (relaxes pelvic ligaments), CRH (promotes fetal cortisol and lung maturation), and thyroid hormone (supports CNS development).
Key Terms & Definitions
- Trophoblast — outer blastocyst cell mass; forms placenta.
- Cytotrophoblast — inner trophoblast layer; cellular and mitotically active.
- Syncytiotrophoblast — outer trophoblast layer; multinucleated, invades endometrium.
- HCG — hormone maintaining corpus luteum and progesterone.
- Chorion — fetal membrane; part of placenta.
- Chorionic villi — finger-like projections for maternal-fetal exchange.
- Cotyledon — placental compartment containing multiple villi.
- Decidua — modified endometrial lining during pregnancy.
- Human placental lactogen (hPL) — hormone altering maternal glucose and fat metabolism.
- TORCH infections — pathogens that cross the placenta (Toxoplasmosis, Others, Rubella, CMV, Herpes).
Action Items / Next Steps
- Review embryology notes on early cell division and implantation.
- Summarize placental functions and hormones for quick reference.
- Study clinical implications of abnormal placental attachment and TORCH infections.