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osteocyte
An osteocyte, a star-shaped cell, is the most commonly found cell in mature bone, and can live as long as the organism itself.〔 The adult human body has about 42 billion osteocytes. Osteocytes have an average half life of 25 years, they do not divide, and they are derived from osteoprogenitors, some of which differentiate into active osteoblasts.〔 Osteoblasts/osteocytes develop in mesenchyme.
In mature bone, osteocytes and their processes reside inside spaces called lacunae (Latin for a ''pit'') and canaliculi, respectively.〔Tate ML, Adamson JR, Tami AE, Bauer TW, (2004). "Cells in Focus, The osteocyte". ''The International Journal of Biochemistry and Cell Biology'' (36), 1-8.〕 When osteoblasts become trapped in the matrix that they secrete, they become osteocytes. Osteocytes are networked to each other via long cytoplasmic extensions that occupy tiny canals called canaliculi, which are used for exchange of nutrients and waste through gap junctions.
Although osteocytes have reduced synthetic activity and (like osteoblasts) are not capable of mitotic division, they are actively involved in the routine turnover of bony matrix, through various mechanosensory mechanisms. They destroy bone through a rapid, transient (relative to osteoclasts) mechanism called osteocytic osteolysis. Hydroxyapatite, calcium carbonate and calcium phosphate is deposited around the cell.
==Morphology==
Osteocytes have a stellate shape, approximately 7 micrometers deep and wide by 15 micrometers in length.〔Sugawara, Y, Kamioka, H, Honjo, T, Tezuka, K, Takano-Yamamoto, T, (2005). "Three dimensional reconstruction of chick calvarial osteocytes and their cell processes using confocal microscopy". ''Bone'' 36(5):877-83.〕 The cell body varies in size from 5-20 micrometers in diameter and contain 40-60 cell processes per cell,〔Tanaka-Kamioka,K, Kamioka H, Ris, H, Lim, SS, (1998). "Osteocyte shape is dependant on actin filaments and osteocyte processes are unique actin-rich projections". ''J. Bone Miner. Res.'' 13(10):1555-68.〕 with a cell to cell distance between 20-30 micrometers.〔 A mature osteocyte contains a single nucleus that is located toward the vascular side and has one or two nucleoli and a membrane.〔 The cell also exhibits a reduced size endoplasmic reticulum, Golgi apparatus and mitochondria, and cell processes that radiate towards the mineralizing matrix.〔Dudley, HR,and Spiro, D, (1961). "The fine structure of bone cells". ''The Journal of Biophysical and Biochemical Cytology'' 11, 627-649.〕 Osteocytes form an extensive connecting syncitial network via small cytoplasmic/dendritic processes in canaliculi.〔Noble,SN (2008). "The osteocyte lineage". ''Archives of Biochemistry and Biophysics'' 473, 106-111.〕
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