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Uptake of marker proteins by glycoprotein-containing cells of the pregnant rat uterus and placenta.

A study was made of cells in the pregnant rat uterus and placenta known to contain glycoprotein inclusions to investigate their ability to endocytose marker proteins (fluorescein conjugated serum and horseradish peroxidase) injected into the maternal circulation. The visceral endoderm showed marked uptake of both proteins, though in later pregnancy this was restricted to the area of yolk sac adjacent to the chorio-allantoic placenta. The intracellular distribution of the endocytosed marker proteins resembled that of the glycoprotein inclusions. In the earlier stages some of the giant cell inclusions contained glycoprotein, some showed staining with the Dunn-Thompson technique for haemoglobin, and some showed peroxidase activity. There was endocytosis of both marker proteins by giant cells, and apparently this occurred independently of ingestion of red blood cells. Uptake by the giant cells persisted to a later stage in the area round the margin of the chorio-allantoic placenta than in the other giant cells. Endocytosis occurred in the labyrinthine trophoblast, and the glycoprotein inclusions found in this situation may represent material being transmitted or digested. The glycoprotein-containing granulated metrial gland cells showed no evidence of endocytotic activity, but there was uptake of both marker proteins by the associated stromal cells.