Development of chrionic villi and hemoplacental barrier

Chorionic villi are finger-like projections of the chorion that invade the maternal decidua and establish the placental exchange interface.

The hemoplacental barrier is the structure separating maternal blood from fetal blood, allowing efficient exchange of nutrient, gas, and waste while minimizing direct blood mixing.

Understanding villous development is crucial for normal fetal growth and placental function.

Formation and Development of Chorionic Villi

Primary Villi (Day 13–15)

• Formed by proliferation of cytotrophoblast into the syncytiotrophoblast
  

  cytotrophoblasts and syncytiotrophoblasts
• Core: cytotrophoblast only, no mesoderm yet
• Appear as small projections on the chorionic surface

Secondary Villi (End of 2nd week)

• Extraembryonic mesoderm invades the cytotrophoblast core of primary villi
• Core becomes mesenchymal, forming connective tissue

Tertiary Villi (3rd week onwards)

• Mesenchymal cells differentiate into blood vessels and connective tissue
• Formation of fetal capillaries within villi
• Villi become functional units of maternal-fetal exchange

Villi Branching

• Villi branch and enlarge, increasing surface area for exchange
• Ending wih two types:
  • Stem villi: supportive, central core
  • Terminal villi: highly branched, site of nutrient and gas exchange

Histology of Chorionic Villi

• Trophoblast layers:
  • outer Syncytiotrophoblast: outer multinucleated(syncytium) layer in direct contact with maternal blood
  • inner Cytotrophoblast: inner layer of mononuclear cells
• Mesenchymal core: connective tissue and fetal capillaries
• Mature terminal villi: thinner barrier for easy exchange
• Stroma: contains fibroblasts and macrophages (Hofbauer cells)

Hemoplacental Barrier

function

Seperate maternal blood from fetal blood minimizing direct blood contact while allowing exchange of nutrient, gas, and waste between.

Structure

1. Syncytiotrophoblast
2. Cytotrophoblast
3. Fetal mesenchyme
4. Endothelium of fetal capillaries

Functional Changes as it develops

• gestation: barrier is thicker, limiting diffusion
• By term: thinner (1–2 μm) in terminal villi, maximizing exchange efficiency
• Transport: gases, nutrients, waste, antibodies

conclusion

Chorionic villi develop in three stages (primary, secondary, tertiary) with invasion of mesenchyme and formation of fetal capillaries.

The hemoplacental barrier consists of trophoblast layers, fetal connective tissue, and endothelium, and its thickness decreases during gestation to facilitate efficient maternal-fetal exchange.

Sources

1. Sadler, T. W. Langman’s Medical Embryology, 14th edition
2. Junqueira, L. C., Carneiro, J. Basic Histology: Text and Atlas, 15th edition