The osteoclasts are multi-nucleated cells that contain numerous mitochondria and lysosomes. These are the cells responsible for the resorption of bone. Attachment of the osteoclast to the osteon begins the process. The osteoclast then induces an infolding of its cell membrane and secretes collagenase and other enzymes important in the resorption process. High levels of calcium, magnesium, phosphate and products of collagen will be released into the extracellular fluid as the osteoclasts tunnel into the mineralized bone. During childhood, bone formation exceeds resorption, but as the aging process occurs, resorption exceeds formation.
Bone resorption is stimulated or inhibited by signals from other parts of the body, depending on the demand for calcium.
Calcium-sensing membrane receptors in the parathyroid gland monitor calcium levels in the extracellular fluid. Low levels of calcium stimulates the release of parathyroid hormone (PTH) from chief cells of the parathyroid gland. In addition to its effects on kidney and intestine, PTH also increases the number and activity of osteoclasts to release calcium from bone, and thus stimulates bone resorption.
High levels of calcium in the blood, on the other hand, leads to decreased PTH release from the parathyroid gland, decreasing the number and activity of osteoclasts, resulting in less bone resorption.