Although osteocytes are relatively inert cells, they are capable of molecular synthesis and modification, as well as transmission of signals over long distances, in a way similar to the nervous system. They are the most common cell type in bone (31,900 mm-3 in bovine bone to 93,200 mm-3 in rat bone). Most of the receptor activities that play an important role in bone function are present in the mature osteocyte. Osteocytes contain glutamate transporters that produce nerve growth factors after bone fracture, which provides evidence of a sensing and information transfer system. When osteocytes were experimentally destroyed, the bones showed a significant increase in bone resorption, decreased bone formation, trabecular bone loss, and loss of response to unloading.
Osteocytes are thought to be mechanosensor cells that control the activity of osteoblasts and osteoclasts within a basic multicellular unit (BMU), a temporary anatomic structure where bone remodeling occurs. Osteocytes generate an inhibitory signal that is passed through their cell processes to osteoblasts for recruitment to enable bone formation.
Osteocyte specific proteins such as sclerostin have been shown to function in mineral metabolism, as well as other molecules such as PHEX, DMP-1, MEPE, and FGF-23, which are highly expressed by osteocytes and regulate phosphate and biomineralization.
The osteocyte is an important regulator of bone mass and a key endocrine regulator of phosphate metabolism.
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