osteocyte

Osteocytes are mature, long-lived bone cells that reside within the mineralized bone matrix. They are derived from osteoblasts that become entrapped during bone formation and subsequently occupy lacunae. Each osteocyte extends cytoplasmic processes through tiny channels called canaliculi, forming an extensive lacuno-canalicular network that connects with other osteocytes and with cells on the bone surface. Osteocytes are the most abundant cell type in mature bone and are found in both cortical and trabecular bone.

During osteogenesis, osteoblasts synthesize organic matrix and promote mineral deposition. As they become surrounded by bone matrix, some osteoblasts differentiate into osteocytes, retracting their processes and maintaining cell-cell connections via gap junctions, primarily formed by connexin 43.

Osteocytes regulate bone remodeling and mineral homeostasis. They sense mechanical strain and fluid flow within the lacuno-canalicular network and coordinate adaptive responses by signaling to osteoblasts and osteoclasts. They secrete signaling molecules such as sclerostin, which inhibits bone formation by osteoblasts; they also express RANKL and osteoprotegerin (OPG) to modulate osteoclast activity, and release fibroblast growth factor 23 (FGF23) to regulate phosphate metabolism and vitamin D activation.

In addition, osteocytes help regulate mineral homeostasis by exchanging phosphate and calcium via gap junctions and by influencing mineral deposition and resorption during remodeling.

Dysfunction of osteocytes or their signaling pathways is linked to osteoporosis, osteogenesis imperfecta, and other metabolic bone diseases. Therapeutic strategies targeting sclerostin, such as romosozumab, aim to enhance bone formation and strength.