A cytological study of plasma membrane modifications, intercellular junctions, and endocytic activity of amniotic epithelium

The purpose of this study was to examine some cytological features of guinea pig amniotic epithelium which may be related to epithelial permeability. In addition to fine structural studies, the amnion was stained with ruthenium red to demonstrate the glycocyalyx and exposed to an exogenous protein (peroxidase) to ascertain some of the permeability characteristics of the intercellular junctions. The apical microvillous border had a distinct glycocalyx of filaments 3‐5 nm in diameter and up to 0.3 μm in length. These filaments stained with ruthenium red as did a narrower zone of glycocalyx adjacent to the outer leaflet of the plasma membrane. Epithelial cells were joined laterally by numerous desmosomes and by tight junctions; no evidence of a classical junctional complex was observed. When cells were exposed to ruthenium red or peroxidase, the tracer was impeded in its penetration between cells, probably by tight junctions. The presence of tight junctions indicates that the intercellular space is not continuous from the amniotic cavity to the underlying connective tissue, and undoubtedly the junctions play a role in controlling transepithelial permeability. The experiments with peroxidase also demonstrated the capacity of the cells to endocytose protein, especially at their lateral and basal surfaces. Late in gestation, increasing numbers of cells underwent cytological changes similar to cells undergoing keratinization. These changes included the disappearance of most of the cell organelles and the thickening of the plasma membrane by the addition of an electron‐dense layer to the inner (cytoplasmic) leaflet. The apparent keratinization of cells may result from increased abrasive trauma caused by the fetus or may be related to accumulation of various waste products in the amniotic fluid. In any event, the cytological modifications, especially of the plasma membrane, may indicate a change in cell permeability properties late in gestation.