Intercellular junctions in the human fetal membranes. A freeze-fracture study.

Freeze-fracture replicas of the human reflected and placental amnion and chorion laeve at term were studied in order to give a systematic survey of the nature and extension of the intercellular junctions in the fetal membranes. No differences could be detected between the reflected and placental amniotic epithelium. In both the replicas never displayed plasma membrane differentiations typical of occluding junctions, while communicating junctions were occasionally and desmosomes frequently seen. In the chorionic trophoblast maculae occludentes, communicating junctions and desmosomes were regularly encountered. It is assumed that the maculae occludentes are remnants of occluding junctions which early in gestation possibly seal off the chorionic cavity; it appears improbable that they contribute significantly to the permeability properties of the chorionic trophoblast, since it is known from previous ultrastructural studies that large open intercellular channels cross the chorionic trophoblast. Thus the absence of occluding junctions, which could act as effective permeability barriers, in both epithelial components of the fetal membranes suggests that the factors able to influence the amniotic fluid turnover or the paraplacental protein exchange are the geometrical relationships and physico-chemical properties of the intercellular channels in the amniotic epithelium and chorionic trophoblast. In addition, communicating junctions were present between fibroblasts in the chorion laeve but not in the amnion, possibly indicating differences in the functional state of these cells and/or their extracellular microenvironments.