Bone structure, formation, and turnover

The skeleton is the body s principal reservoir of calcium and phosphorus. Contrary to its appearance, bone is a dynamic tissue, and calcium and phosphate are continuously deposited and released. Bone is a modified connective tissue consisting of a cellular component, an organic matrix and an inorganic (mineral) phase. Its cells are osteoblasts, osteoclasts, osteocytes, and osteoprogenitor cells. The last are a type of mesenchymal cell that can differentiate into any of the other three types and to which the other types can revert. 

As mineralization progresses, osteoblasts become surrounded by growing bone and differentiate into osteocytes, which reside in individual lacunae in the bone. These lacunae communicate with each other via canaliculi, exchanging substrates and metabolites. Osteocytes nourish the bone, which is a living, highly vascularized tissue. The bone cells account for 2-3% of mature bone volume. Bone mass is about 65% mineral and 35% organic matrix. 

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