An osteoclast (from Ancient Greek ὀστέον (osteon) 'bone' and κλαστός (clastos) 'broken') is a type of bone cell that removes bone tissue. This function is critical in the maintenance, repair, and remodeling of bones of the vertebral skeleton. The osteoclast disassembles and digests the composite of hydrated protein and mineral at a molecular level by secreting acid and a collagenase, a process known as bone resorption. This process also helps regulate the level of blood calcium.
Osteoclasts are found on those surfaces of bone that are undergoing resorption. On such surfaces, the osteoclasts are seen to be located in shallow depressions called resorption bays (Howship's lacunae). The resorption bays are created by the erosive action of osteoclasts on the underlying bone. The border of the lower part of an osteoclast exhibits finger-like processes due to the presence of deep infoldings of the cell membrane; this border is called ruffled border. The ruffled border lies in contact with the bone surface within a resorption bay. The periphery of the ruffled border is surrounded by a ring-like zone of cytoplasm, which is devoid of cell organelles but rich in actin filaments. This zone is called the clear zone or sealing zone. The actin filaments enable the cell membrane surrounding the sealing zone to be anchored firmly to the bony wall of Howship's lacunae. In this way, a closed subosteoclastic compartment is created between the ruffled border and the bone that is undergoing resorption. The osteoclasts secrete hydrogen ions, collagenase, cathepsin K and hydrolytic enzymes into this compartment. Resorption of bone matrix by the osteoclasts involves two steps: (1) dissolution of inorganic components (minerals), and (2) digestion of organic component of the bone matrix. The osteoclasts pump hydrogen ions into the subosteoclastic compartment and thus create an acidic microenvironment, which increases solubility of bone mineral, resulting in the release and re-entry of bone minerals into the cytoplasm of osteoclasts to be delivered to nearby capillaries. After the removal of minerals, collagenase and gelatinase are secreted into the subosteoclastic compartment. These enzymes digest and degrade collagen and other organic components of decalcified bone matrix. The degradation products are phagocytosed by osteoclasts at the ruffled border. Because of their phagocytic properties, osteoclasts are considered to be a component of the mononuclear phagocyte system (MPS). The activity of osteoclasts is controlled by hormones and cytokines. Calcitonin, a hormone of the thyroid gland, suppresses osteoclastic activity. Osteoclasts do not have receptors for parathyroid hormone (PTH). However, PTH stimulates osteoblasts to secrete a cytokine called osteoclast-stimulating factor, which is a potent stimulator of osteoclastic activity.
