Negative charges bound to collagen fibrils in the rabbit articular cartilage: A light and electron microscopic study using cationic colloidal iron

Negative charged sites in the normal rabbit articular cartilage were investigated using cationic colloidal iron methods. In light microscopy of the cartilage stained with the colloidal iron at pH 1.5, a distinct Prussian blue reaction was observed in the pericellular matrix, and a weak blue reaction in the territorial and interterritorial matrices. At pH 7.0, a diffuse Prussian blue reaction was observed in the pericellular and interterritorial matrices. Digestion with chondroitinase ABC, hyaluronidase and keratanase could not erase the Prussian blue reaction. However, the sections digested with collagenase followed by chondroitinase ABC showed significant elimination of the Prussian blue reaction. Electron microscopy of ultrathin sections stained with the colloidal iron at pH 1.5 revealed that the cationic colloid particles were deposited abundantly in the pericellular matrix and dotted along collagen fibrils in the territorial and interterritorial matrices. The present results suggest that negatively charged sites in the articular cartilage derive mostly from chondroitin sulfate, whose proteoglycans firmly bind to collagen fibrils. Such an ultrastructure may maintain the electrostatic microenvironment in the collagen plexus, holding much water in the cartilage matrix, and also producing biomechanical properties such as tensile strength and elasticity of the cartilage.