Electron microscopy of ultrathin sections stained with cationic iron colloid revealed that, in the choroid plexus of the rat brain ventricles, the luminal surface and fenestral diaphragm of the capillary endothelium as well as the basement membranes of the endothelium and epithelium are strongly anionic or intensely negatively charged. The iron colloid reaction to these anionic sites was erased by treatment with hyaluronidase or digestions with chondroitinase ABC/heparitinase/keratanase. These results indicate that sulfated proteoglycans provide such anionic sites of the choroidal capillaries. Discussion suggested that the negative charge on the luminal surface of the capillary endothelium prevents the adhesion of blood cells to capillary walls and also prevents endothelial adhesion by their repelling each other. It was further discussed that the negatively charged endothelial fenestrae and basement membranes may act as a charge barrier to inhibit the passage of anionic molecules.
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