Bone-Orchestrating Cells, Osteocytes

概要

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Osteocytes build up functional syncytia, i.e., the osteocytic lacunar-canalicular system（OLCS）. The osteocytes are interconnected through gap junctions between their cytoplasmic processes, which pass through narrow passageways referred to as osteocytic canaliculi. There are two possible ways, in which molecules can be transported throughout the OLCS: via the cytoplasmic processes and their gap junctions, and via the pericellular space in the osteocytic canaliculi. Transport of minerals and small molecules through a spatially well-organized OLCS appears to be pivotal for bone mineral homeostasis and bone remodeling control. Recently, osteocyte-derived molecules -- sclerostin, dentin matrix protein-1, fibroblast growth factor 23（FGF23）-- have been put in evidence as they may be related to osteocytic functions such as regulation of bone remodeling and so forth. Osteocytes were shown to regulate phosphorus serum levels and osteoblastic activity through the expression of FGF23 and sclerostin. In our observations, FGF23 and sclerostin synthesis seemed to be associated with the spatial regularity of the OLCS: both proteins were consistently expressed by osteocytes in epiphyses and cortical bones showing regularly arranged OLCS. In contrast, mice bearing high bone turnover, e.g., osteoprotegerin deficient mice, revealed markedly-diminished sclerostin. This review will introduce our recent studies on the regularity of OLCS and the osteocytic function.