Lecture on the Development of the Placenta

Jul 23, 2024

Development of the Placenta

Overview

  • Discuss development of the placenta
  • Hormonal functions of the placenta
  • Clinical correlations
  • Reference video on cleavage and implantation

Implantation and Early Development

  • Fertilization: Sperm + secondary oocyte (metaphase 2) = zygote
  • Cleavage Stages: Zygote divides --> 2-cell stage --> 4-cell stage --> 8-cell stage --> 16-cell (morula)
  • Morula: Hollow ball of 16+ cells with a zona pellucida (ZP)
  • Blastocyst Formation: Water enters morula, forming a blastocoel --> Differentiates into outer cell mass (trophoblast) and inner cell mass (embryoblast)
  • Implantation: Blastocyst attaches to the endometrium, which later differentiates into decidua (decidua basalis and decidua functionalis)

Detailed Implantation Process

  1. Attachment: Loose connection
    • Trophoblast microvilli adhere to endometrial pinopods
  2. Stronger attachment: Integrins on trophoblast bind to selectins and collagen on endometrium
    • Chemokines stabilize adhesion
  3. Differentiation of Trophoblasts: CytoTrophoblast & SynCytioTrophoblast
    • CytoTrophoblast: Proliferates and contributes to syncytiotrophoblast
    • SynCytioTrophoblast: Releases hydrolytic enzymes and HCG, ensuring endometrial integrity
  4. Syncytiotrophoblast Invasion: Releases enzymes to invade stromal tissue
  5. HCG Secretion: Maintains progesterone levels to prevent menstruation until placenta takes over

Villous Development

  • Primary Villi: Cytotrophoblast penetrates syncytiotrophoblast, forming outer cytotrophoblastic shell
  • Secondary Villi: Extraembryonic mesoderm invades primary villi
  • Tertiary Villi: Mesoderm forms blood vessels, allowing for nutrient and gas exchange

Formation of Chorionic Structures

  • Extraembryonic Coelom: Forms around Day 12, becoming chorionic cavity
  • Chorionic Plate: Develops from extraembryonic mesoderm
  • Outer Cytotrophoblastic Shell: Formed by cytotrophoblast penetrating syncytiotrophoblast
  • Formation of Connecting Stalk: Converts into the umbilical cord with arteries and veins

Maternal-Fetal Exchange and Circulation

  • Blood Flow Route: Umbilical arteries > Chorionic arteries > Cotyledon arteries > Capillaries in villi
  • Gas and Nutrient Exchange: Takes place between intervillous spaces filled with maternal blood and fetal blood vessels

Placental Development Timeline

  • Weeks 4-8: Tertiary villi become more branched for increased surface area
  • 4th-5th Month: Cotyledon formation due to septations of decidua
  • >20 Weeks: Cytotrophoblastic layer regresses, enhancing exchange efficiency

Placental Linings and Associated Structures

  • Layers: Amniotic membrane, chorionic membrane, decidua capsularis, decidua parietalis
  • Placental Formation: Chorionic frondosum (fetal component) interacts with decidua basalis (maternal component)
  • Fusion: Decidua capsularis and decidua parietalis fuse, obliterating the uterine cavity

Hormonal Functions of the Placenta

  • Estrogen & Progesterone: Maintain endometrial lining, increase secretions, and support fetal development
  • Human Chorionic Gonadotropin (HCG): Maintains corpus luteum function
  • Thyroid Hormone: Supports CNS development
  • Human Placental Lactogen: Modifies maternal metabolism to support fetal needs (increases glucose and fatty acid availability)
  • Relaxin: Relaxes pubic symphysis ligaments
  • Corticotropin-Releasing Hormone (CRH): Promotes cortisol production for lung development and surfactant production

Metabolic and Exchange Functions

  • Gas Exchange: Oxygen to fetus, CO2 removal
  • Nutrient Delivery: Glucose, amino acids, fatty acids, water-soluble vitamins, IgG antibodies (passive immunity)
  • Waste Removal: Urea, uric acid

Pathogenic Concerns (TORCH Infections)

  • Toxoplasmosis: Toxoplasma gondii
  • Others: HIV, syphilis, hepatitis B
  • Rubella: Can cause cardiac and congenital defects
  • CMV (Cytomegalovirus)
  • Herpes Simplex Virus: Type 2
  • Zika Virus: Vertical transmission risk

Clinical Correlations

  • Placenta Accreta, Increta, Percreta: Abnormal placenta attachment leading to complications
  • Rhesus (Rh) Incompatibility: Condition where Rh-negative mother has immune response against Rh-positive fetal blood cells

Treatment: RhoGAM to prevent hemolytic disease