Fine structure of the placental labyrinth in the rat at term and during prolonged gestation.

The fine structure of the placental labyrinth in the rat was described at term (Day 22) and during prolonged gestation (Day 23, 24 and 25). Gestation was prolonged by dialy subcutaneous injection of progesterone to the mothers from Day 20 through Day 24 of pregnancy. The placental labyrinth at term consisted of maternal and foetal blood spaces separated by three layers of trophoblast and an endothelium. During prolonged gestation (Day 24 and 25 only) the layers of trophoblast exhibited discontinuity, loss of cytoplasmic elements and the presence of fibronoid material. In addition ot the morphological observations, it was found that foetal blood glucose levels on Day 24 and 25 were significantly lower than values at term. The results were discussed in reference to placental insufficiency and its effect on the foetus.