<P> Bone reabsorption is resorption of bone tissue, that is, the process by which osteoclasts break down the tissue in bones and release the minerals, resulting in a transfer of calcium from bone tissue to the blood . </P> <P> The osteoclasts are multi-nucleated cells that contain numerous mitochondria and lysosomes . These are the cells responsible for the resorption of bone . Osteoclasts are generally present on the outer layer of bone, just beneath the periosteum . 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 . Osteoclasts are prominent in the tissue destruction found in psoriatic arthritis and rheumatological disorders . </P> <P> The human body is in a constant state of bone remodeling . Bone remodelling is a process which maintains bone strength and ion homeostasis by replacing discrete parts of old bone with newly synthesized packets of proteinaceous matrix . Bone is resorbed by osteoclasts, and is deposited by osteoblasts in a process called ossification . Osteocyte activity plays a key role in this process . Conditions that result in a decrease in bone mass can either be caused by an increase in resorption or by a decrease in ossification . During childhood, bone formation exceeds resorption . As the aging process occurs, resorption exceeds formation . </P> <P> Bone resorption rates are much higher in post-menopausal older women due to estrogen deficiency related with menopause . Common treatments include drugs that increase bone mineral density . Bisphosphonates, RANKL inhibitors, SERMs--selective oestrogen receptor modulators, hormone replacement therapy and calcitonin are some of the common treatments . Light weight bearing exercise tends to eliminate the negative effects of bone resorption . </P>

The continued erosion and replacement of bone tissue is called