Osteocyte
A mature bone cell — a former osteoblast entombed within mineralised matrix in a lacuna — that maintains the matrix and acts as the mechanosensor and endocrine regulator of bone.
Overview
Osteocytes are the most numerous bone cells (≈90–95%; ≈40 billion in an adult). Each lies in a lacuna and extends ≈50 slender dendritic processes through canaliculi to contact neighbouring osteocytes, lining cells and the vascular surfaces through gap junctions — a lacunocanalicular network comparable in complexity to the neuronal network. Fluid flow in this network during loading is sensed by the cells (via cilia, integrins and the processes themselves).
Osteocytes are the conductors of remodelling. With loading they reduce secretion of sclerostin (SOST), an inhibitor of Wnt signalling, allowing osteoblastic bone formation; with disuse sclerostin rises and bone is lost. Damaged or dying osteocytes (after microcracks) release RANKL, targeting osteoclasts to the damaged site. Osteocytes are also endocrine cells: they secrete FGF23, which promotes renal phosphate excretion and suppresses calcitriol synthesis. They can release calcium from the perilacunar matrix (osteocytic osteolysis). Osteocytes may live for decades; empty lacunae indicate bone death (osteonecrosis).
- Location
Lacunae between lamellae of compact and spongy bone.
Anatomy
- Histology
Small, flattened cells with condensed nuclei in almond-shaped lacunae; canaliculi visible in ground sections.
Function
- Maintain bone matrix
- Sense mechanical strain and translate it into remodelling signals (sclerostin, RANKL)
- Regulate phosphate homeostasis via FGF23
- Detect and signal microdamage
Clinical significance
Osteonecrosis (avascular necrosis)
Loss of blood supply kills osteocytes (empty lacunae on histology) — e.g. femoral head after intracapsular fracture or steroid use, proximal scaphoid, talus, lunate (Kienböck disease).
Romosozumab
A monoclonal antibody against osteocyte-derived sclerostin; increases bone formation and decreases resorption in severe osteoporosis.
Test yourself
Which cells are primarily responsible for sensing mechanical load and regulating bone formation through secretion of sclerostin?
- Osteoblasts
- Osteoclasts
- Osteocytes
- Osteoprogenitor cells
Answer: Osteocytes
Osteocytes, connected via canaliculi, are the mechanosensors of bone. Loading reduces their secretion of sclerostin (a Wnt antagonist), releasing osteoblast activity; disuse increases sclerostin and bone loss. Romosozumab is an anti-sclerostin antibody.
Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.