Fetal Membranes II Anatomy Unit

Development of Chorion

• Chorion develops during the second week of gestation.
• Consists of three layers: syncytiotrophoblast, cytotrophoblast, and extraembryonic mesoderm (somatic layer).

Chorionic Villi

• Finger-like projections from the trophoblast into decidua basalis and capsularis.
• Types of chorionic villi based on structure:
  • Primary Villi: Composed of outer syncytiotrophoblast and inner cytotrophoblast.
  • Secondary Villi: Include outer syncytiotrophoblast, inner cytotrophoblast, and extraembryonic mesoderm.
  • Tertiary Villi: Incorporate outer syncytiotrophoblast, inner cytotrophoblast, extraembryonic mesoderm, and fetal blood vessels.

Fate of Chorion

• After the 8th week, chorion changes due to villi degeneration and growth:
  • Villi facing decidua capsularis compress and form chorion leave.
  • Villi facing decidua basalis increase in number to form chorion frondosum.

Placenta Structure and Barrier

• Acts as a barrier between maternal and fetal blood.
• Before 20 weeks: Consists of syncytiotrophoblast, cytotrophoblast, extraembryonic mesoderm, and fetal capillary endothelium.
• After 20 weeks: Comprised of syncytiotrophoblast, extraembryonic mesoderm, and fetal capillary endothelium.
• Late pregnancy: Syncytiotrophoblast and fetal endothelium form the barrier separating maternal and fetal blood.

Placenta Overview

• Primary site for nutrient and gas exchange between mother and fetus.
• Fetal part: chorion frondosum; maternal part: decidua basalis and maternal blood in the intervillous space.
• Gross characteristics: discoid shape, diameter of 15-20 cm, thickness of 2-3 cm, and weight of 500-600 grams.

Placenta Surfaces

• Fetal Surface: Smooth, shiny, with umbilical cord attachment; covered by amnion.
• Maternal Surface: Cobblestone appearance.

Functions of the Placenta

• Exchange Functions:
  • Respiratory: Gas exchange (O2 and CO2).
  • Nutritive: Nutrients and electrolytes transfer.
  • Excretory: Removal of waste products.
• Antibody Transmission: Maternal antibodies for certain infections, but some viruses can infect the fetus.
• Barrier Function: Prevents certain bacteria and drugs from reaching fetoplacental circulation.
• Endocrine Functions: Produces various hormones including:
  • Protein hormones: Human chorionic gonadotropin, human placental lactogen, relaxin.
  • Steroid hormones: Progesterone (from month 4), estrogen (enhances myometrium sensitivity to oxytocin).

Anomalies of Placenta

• Anomalies in position include conditions like placenta previa.

This quiz focuses on the development and functions of fetal membranes, specifically the chorion and placenta. It covers topics such as the fate of chorionic villi and common anomalies associated with the placenta. Engage in detailed case scenarios to apply your knowledge of fetal anatomy.