Electron microscopy of ultrathin sections stained with cationic iron colloid revealed that the rat pineal gland is provided with wide and intensely negative-charged pericapillary spaces. Light microscopically, the negative charging of the pericapillary spaces was completely eliminated by digestion with hyaluronidase and chondroitinase ABC. This pericapillary negative charging was also erased by digestion with collagenase. The results indicate that the negative charging is derived from sulfated proteoglycans which are bound to collagen molecules. These sulfated proteoglycans in the pericapillary spaces may retain numerous water molecules to form a tissue gel, and so act as a selective sieve regulating the passage of tissue molecules.
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