Morphological aspects of the biological function of the osteocytic lacunar canalicular system and of osteocyte-derived factors

Osteocytes are organized in functional syncytia collectively referred to as 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 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 is vital for bone mineral homeostasis, mechanosensing, 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 mechanosensing, regulation of bone remodeling, and so forth. FGF23 regulates serum phosphate concentration by affecting renal function, while sclerostin can inhibit osteoblastic activities. In our observations, FGF23 and sclerostin synthesis seemed to be associated with the spatial regularity of the OLCS. This review will introduce our recent morphological studies on the regularity of OLCS and the synthesis of osteocyte-derived FGF23 and sclerostin.

▸ Osteocytes act as a functional group (OLCS), since their cytoplasmic processes are connected through gap junctions. ▸ Intact OLCS seems crucial for mineral transport among osteocytes, and may be important for bone remodeling. ▸ Ablation of osteocytes resulted in broadly demineralized areas in the periphery of the osteocytic lacunae. ▸ Osteocytes synthesize FGF23 regulating serum concentration of phosphate by co-operating with kidney. ▸ Deficiency of klotho, a co-receptor for FGF23-klotho axis, disrupted osteocytic function.