Mechanism of osteoclast mediated bone resorption

The osteoclast is the main bone resorbing cell. It has several structural features which are related to its function. In active resorption stage this multinucleated giant cell shows remarkable polarity which, with its basolateral and apical membrane surfaces, resembles a secreting epithelial cell. The third specialized membrane area, sealing zone, mediates the attachment of the osteoclast to the bone surface. The rate of bone resorption can be regulated either by changing the number or the activity of resorbing osteoclasts. These processes can be influenced by different systemic circulating factors as well as local, bone matrix or other bone cell-derived factors. After activation of stem cells, preosteoclasts are guided to the bone surface where they undergo fusion into multinucleated osteoclasts. A special type of cell attachment to mineralized bone surface precedes actual resorption process. During cell attachment cytoskeletal structures are organized into a typical belt-like structure. The actual attachment receptor as well as its counterpart in bone matrix are not yet known. Bone mineral dissolution is then initiated by active secretion of H+ through the ruffled border membrane area. Acidification of the resorption lacuna together with hydrolytic enzymes completes finally the degradation of organic matrix of bone.