Mechanism of bone turnover.

Bone remodelling is a cellular mechanism behind the bone turnover. It renews the old bone piece by piece and thus ensures the correction of possible microdamage and enables the regulation of mineral homeostasis. The basic mechanism of bone remodelling is similar in all types of bone and includes the resorption of old bone and the formation of equal amount of new bone at the same place. Histomorphometric studies have revealed the cellular details of remodelling and have shown that it is composed by the temporally and spatially regulated action of different bone cells and their precursors. Recent in vitro studies with osteoclasts and osteoblasts have increased our knowledge of the molecular mechanisms of bone remodelling. Molecular characterization of bone matrix proteins have suggested new functions to many of them and thereby increased our possibilities of understanding the local regulation mechanisms of remodelling. Bone matrix has been shown to contain several biologically active compounds which have effects on bone forming and resorbing cells and their precursors. Details of the functional mechanism of osteoclasts are also in the process of being discovered. However, several questions concerning bone remodelling still remain open: the molecular explanation for selection of the remodelling site; the coupling of bone resorption to formation, and the interactions between different cell types during the remodelling cycle.