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Fetal membranes and placenta

The placental barrier

The placental barrier is composed of structures that separate the maternal and the fetal blood .The makeup of the placental barrier changes over the course of the pregnancy.

 

In the first trimester it consists of the syncytiotrophoblast, the cytotrophoblast (Langhans' cells), the villus mesenchyma (in which numerous ovoid Hofbauer cells that exhibit macrophage properties are found) and the fetal capillary walls.

Fig. villosite01 - Villus in the first trimenon
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  1. Intervillous space
  2. Syncytiotrophoblast
  3. Cytotrophoblast
  4. Villus mesenchyma
  5. Fetal capillaries
  6. Hofbauer macrophages

Legend
Fig. villosite01

The villus has an intact syncytio- and cytotrophoblast layer. In the villus interior there are mesenchymal cells with macrophages and fetal capillaries.

During the 4th month the cytotrophoblast disappears from the villus wall and the thickness of the barrier decreases while the surface area increases (roughly 12 m2 towards the end of the pregnancy). In the 5th month the fetal vessels have multiplied their branches and gotten closer to the villus surface.

Fig. villosite02 - Villus in the second trimenon
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  1. Intervillous space
  2. Syncytiotrophoblast
  3. Cytotrophoblast
  4. Villus mesenchyma
  5. Fetal capillaries
  6. Hofbauer macrophages

Legend
Fig. villosite02

In the middle third of the pregnancy the capillaries migrate to the villus surface. The cytotrophoblast layer disappears slowly and the syncytiotrophoblast layer becomes thinner.

During the 6th month the nuclei of the syncytiotrophoblast group together in the so-called proliferation knots. The other zones of the syncythiothrophoblast lack nuclei and are adjacent to the capillaries (exchange zones).

Fig. villosite03 - Placental barrier in a mature villus
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  1. Intervillous space (with maternal blood)
  2. Placental barrier of a terminal villus
  3. Fetal capillaries
  4. Merged basal membranes of the fetal capillary and
    of the syncythiothrophoblast
  5. Endothelial cells
  6. Rare cytotrophoblast cells
  7. Basal membrane of the capillaries
  8. Basal membrane of the trophoblast portion
  9. Syncytiotrophoblast with proliferation knots (nuclei rich region)

Legend
Fig. villosite03

At its thinnest part, the placental barrier is reduced to the nucleus-free syncytiotrophoblast, the merged basal membrane and the endothelium.