noun, plural: osteocytes
A stellate, non-dividing osteoblast embedded in mature bony tissue
Bone cells are found within the bone tissues, and they are responsible for the make-up of the skeleton of vertebrates. The four main types of bone cells are the (1) osteoclasts, (2) osteoblasts, (3) osteocytes, and (4) lining cells.
The osteocyte is a non-dividing, stellate bone cell. The stellate shape is due to the presence of cytoplasmic extensions (about 40-60 cell processes) that radiate towards the mineralizing matrix.1, 2 The cell processes of osteocytes occupy the canaliculi and connect to each other. The cell processes are used for the exchange of nutrients and waste via the gap junctions.
The osteocyte is an inactive osteoblast, which is a differentiated, mononucleate cell from where bone develops. An active osteoblast is a bone-forming cell. The osteoblasts act as groups and connect with each other to form bone. They secrete mainly collagen and few other proteins (e.g. osteocalcin and osteopontin), which serve as the initial components of an organic bone matrix. Later, they secrete calcium phosphate, which hardens and forms the mineral, hydroxyapatite in the bone matrix. Osteoblasts that become trapped in their own secretions become the osteocytes. The osteocytes are no longer mitotically active and have reduced synthetic activity. Nevertheless, they are actively involved in osteocytic osteolysis. They are important in the regulation of bone mass, acting as mechanosensor cells that control the activity of osteoblasts and osteoclasts during bone remodeling.
Word origin: osteo- (“bone”) + -cyte (“cell”)
1 Tanaka-Kamioka, K., Kamioka, H., Ris, H., & Lim, S. S. (1998). "Osteocyte shape is dependant on actin filaments and osteocyte processes are unique actin-rich projections". J. Bone Miner. Res. 13 (10): 1555–68.
2 Dudley, H. R., & Spiro, D. (1961). "The fine structure of bone cells". The Journal of Biophysical and Biochemical Cytology 11: 627–649.