Osteocyter

Osteocyter, or osteocytes, are mature bone cells embedded in mineralized bone matrix. They originate from osteoblasts that become encased in bone tissue during formation and reside in lacunae.

Within the bone matrix, osteocyter have long cytoplasmic extensions that extend through narrow channels called canaliculi, forming a lacuno-canalicular network that connects most osteocytes and links them to cells on the bone surface. This network enables intercellular communication and nutrient exchange.

The principal roles of osteocyter are the maintenance of bone tissue and the mechanosensory regulation of bone remodeling. They sense mechanical strain and coordinate activities of osteoblasts and osteoclasts through signaling molecules. Osteocyter secrete sclerostin, a regulator that inhibits bone formation, and modulate osteoclast activity by producing RANKL and osteoprotegerin (OPG). Mechanical loading generally reduces sclerostin expression and favors bone formation, whereas unloading or aging can increase sclerostin and promote bone loss.

In addition to remodeling signaling, osteocyter contribute to systemic mineral homeostasis. They express fibroblast growth factor 23 (FGF23), a hormone that lowers serum phosphate by reducing renal phosphate reabsorption and suppressing activation of vitamin D.

Clinically, osteocyter dysfunction or altered signaling is linked to osteoporosis and other metabolic bone diseases. Research into osteocyte biology informs therapeutic approaches, including inhibitors of sclerostin to stimulate bone formation.