Fluid shift across the placenta: II. Fetomaternal transfer of horseradish peroxidase in the guinea pig.

The fetal side of the isolated guinea-pig chorio-allantoic placenta has been perfused with a fluid containing horseradish peroxidase. Electron micrographs show that protein is transferred across the fetal capillary endothelium via micropinocytosis as well as by transport through intercellular clefts. Under normal hydrostatic pressure no pinocytosis or other signs of fetomaternal protein transfer across the trophoblast have been observed. Instead, macrophages ingest the proteins that have reached the interstitial space between fetal endothelium and trophoblast. When the fetal venous pressure is raised up to 20 mmHg about 30 to 50 per cent of the arterial perfusion fluid, including proteins, is shifted into the maternal lacunae. In the beginning of this process slender tubules, and later on bag-like channels, appear that cross the trophoblast and probably connect lacunae and interstitium. The inner surface of the channels and the maternal lacunar surface are labelled by peroxidase. This indicates a fetomaternal transfer of proteins conveyed by the fluid shift. Protein transfer and fluid shift return to their previous levels when the fetal venous pressure is reduced. The size and number of channels are reduced but their structure and the reversibility of the events refute the possibility that they are of traumatic origin. It cannot be decided whether the channels are pre-existent but unrecognizable, or de-novo formations.