Osteoclasts (top image on the right), which are responsible for bone resorption, are rare cells with only 2-3 cells seen per 1 mm3 of bone. Osteoclasts have a " ruffled border" and are multinucleated. However, the loss of function in osteoclasts, problems with their differentiation, and decrease in their number lead to bone osteosclerosis (abnormal hardening of the bone and an increase in bone density)/osteopetrosis (failure of osteoclasts to resorb bone). On the other hand, an increase in their number or function induces bone osteoporosis.
Osteoblasts (bottom image on the right) are produced by bone cells and are the bone builders. They are responsible for the synthesis and mineralization of bone during both the initial bone formation process and later bone remodeling process. Osteoblasts form a closely packed sheet on the surface of the bone, from which cellular processes extend through the developing bone. They rebuild the skeleton, first by filling in the holes with collagen, and then by laying down crystals of calcium and phosphorus.
Each year, about 10 - 30% of the adult skeleton is remodeled by the work of both osteoclasts and osteoblasts. The balance of bone formation and resorption is controlled by a mix of hormones and biochemicals. If bone resorption occurs at a greater rate than bone build formation, the bone loses density and puts the individual at risk for osteoporosis. The loss of estrogen in post-menopausal women is associated with rapid bone resorption and loss of bone density and therefore puts this population at highest risk for osteoporosis.