Development of chrionic villi and hemoplacental barrier

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Development of Chorionic Villi and the Hemoplacental Barrier

Introduction

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 nutrient, gas, and waste exchange 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
• 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
• Villi are now called secondary villi

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
• Two types:
  • Stem villi: supportive, central core
  • Terminal villi: highly branched, site of nutrient and gas exchange

Histology of Chorionic Villi

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

Hemoplacental Barrier

Structure

• The barrier between maternal blood and fetal capillaries includes:
  1. Syncytiotrophoblast
  2. Cytotrophoblast (discontinuous in late gestation)
  3. Fetal connective tissue (mesenchyme)
  4. Endothelium of fetal capillaries

Functional Changes

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

Clinical Relevance

• Abnormal villous development → miscarriage, intrauterine growth restriction (IUGR)
• Thickened hemoplacental barrier → decreased nutrient/gas exchange
• Villi and barrier histology are important in placental pathology evaluation

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