Extracellular and intracellular digestion of complex carbohydrates by osteoclasts

The role of osteoclasts or chondroclasts in degradation and synthesis of complex carbohydrates was investigated using the high iron diamine-thiocarbohydrazide-silver proteinate method (HID-TCH-SP) for sulfated glycoconjugates and the periodate-thiocarbohydrazide-silver proteinate (PA-TCH-SP) method for vicinal glycol-containing glycoconjugates. HID-TCH-SP stained the calcified cartilage matrix, the osteoclast ruffled border, vacuoles and heterophagosomes but not the Golgi apparatus and primary lysosomes. The size and number of HID-TCH-SP stain deposits progressively decreased from the calcified cartilage matrix to the ruffled border (p less than 0.001). Enzyme digestion with testicular hyaluronidase removed most HID-TCH-SP stain deposits averaging 13 nm. in diameter in the extracellular matrix, cytoplasmic vacuoles, and heterophagosomes of osteoclasts. Only sparse stain deposits averaging 8 nm. in diameter and presumed to be keratan sulfate remained in these sites after enzyme digestion. Osteoclast heterophagosomes contained the highest concentration of hyaluronidase-resistant material suggesting delayed degradation of keratan sulfate at this site. PA-TCH-SP strongly stained collagen fibrils in the calcified cartilage matrix. Reactive collagen fibrils were also observed in extracellular channels but only rarely were identifiable collagen fibrils observed in cytoplasmic vacuoles. A progressive decrease in the diameter of PA-TCH-SP reactive collagen fibrils was observed between the calcified cartilage matrix and the ruffled border region (p less than 0.001). PA-TCH-SP stained cisternae of rough endoplasmic reticulum, Golgi saccules, and primary lysosomes consistent with the synthesis and packaging of glycoprotein enzymes at these sites. These results indicate that the dissolution of sulfated glycoconjugates requires osteoclastic engulfment of degraded material and subsequent intracellular digestion, whereas the dissolution of collagen fibrils appears to be completed extracellularly.