An osteocyte, an oblate-shaped type of bone cell with dendritic processes, is the most commonly found cell in mature bone. It can live as long as the organism itself. Osteocytes do not divide and have an average half life of 25 years. They are derived from osteoprogenitor cells, some of which differentiate into active osteoblasts (which may further differentiate to osteocytes). Osteocytes are simply osteoblasts trapped in the matrix that they secrete. They 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.

Structure

Osteocytes have a stellate shape, approximately 7 micrometers deep and wide by 15 micrometers in length. The cell body varies in size from 5–20 micrometers in diameter and contain 40–60 cell processes per cell, with a cell to cell distance between 20–30 micrometers. Osteocytes form an extensive lacunocanalicular network within the mineralized collagen type I matrix, with cell bodies residing within lacunae, and cell/dendritic processes within channels called canaliculi.

thumb|An osteocyte in rat bone exposed by resin cast etching

Development

The fossil record shows that osteocytes were present in bones of jawless fish 400 to 250 million years ago. Osteocyte size has been shown to covary with genome size; and this relationship has been used in paleogenomic research.

During bone formation, an osteoblast is left behind and buried in the bone matrix as an "osteoid osteocyte", which maintains contact with other osteoblasts through extended cellular processes. Although recently it was shown that vascular smooth muscle cells drive osteocyte differentiation, most aspects of osteocytogenesis remain largely unknown. Various molecules have been reported to be involved. Examples include matrix metalloproteinases (MMPs), dentin matrix protein 1 (DMP-1), osteoblast/osteocyte factor 45 (OF45), Klotho, TGF-beta inducible factor (TIEG), lysophosphatidic acid (LPA), E11 antigen, and oxygen.

thumb|HAADF-STEM electron image of a maturing osteocyte (preosteocyte or osteoid osteocyte) at the bone surface, appearing directly above osteoblast-like precursor cells (decalcified matrix). Note the elongated cell processes that are surrounded by the collagen type I matrix and already crossing lamellar boundaries as collagen (and eventually mineral) continues to entomb the cell. |350x350px

Palumbo et al. (1990) distinguish three cell types from osteoblast to mature osteocyte: type I preosteocyte (osteoblastic osteocyte), type II preosteocyte (osteoid osteocyte), and type III preosteocyte (partially surrounded by mineral matrix). The cell undergoes a dramatic transformation from a polygonal shape to a cell that extends dendrites toward the mineralizing front, followed by dendrites that extend to either the vascular space or bone surface. As the osteoblast transitions to an osteocyte, alkaline phosphatase is reduced, and casein kinase II is elevated, as is osteocalcin. Oxygen tension may regulate the differentiation of osteoblasts into osteocytes, and osteocyte hypoxia may play a role in disuse-mediated bone resorption. Osteocytes generate an inhibitory signal that is passed through their cell processes to osteoblasts for recruitment to enable bone formation.

Osteocytes are also a key endocrine regulator in the metabolism of minerals such as phosphates. Only osteocytes express sclerostin, which acts in a paracrine fashion to inhibit bone formation.

Clinical significance

Clinically important research of gel based in vitro 3D model for the osteocytic potentiality of human CD34+ stem cells has been described. The results confirm that the human CD34+ stem cells possess unique osteogenic differentiation potential and can be used in the early regeneration of injured bone. Osteocytes die as a consequence of senescence, degeneration/necrosis, apoptosis (programmed cell death), and/or osteoclastic engulfment. Osteocyte apoptosis is thought to be related to decreased mechanotransduction, which possibly leads to the development of osteoporosis. Apoptotic osteocytes release apoptotic bodies expressing RANKL to recruit osteoclasts.

Osteocyte cell death can occur in association with pathologic conditions such as osteoporosis and osteoarthritis, which leads to increased skeletal fragility, linked to the loss of ability to sense microdamage and/or signal repair. Oxygen deprivation that occurs as the result of immobilization (bed rest), glucocorticoid treatment, and withdrawal of oxygen have all been shown to promote osteocyte apoptosis.

See also

  • List of human cell types derived from the germ layers
  • List of distinct cell types in the adult human body

References

  • – "Cartilage and Bone and Bone Histogenesis: cells of* – "Cartilage and Bone and Bone Histogenesis: compact bone"* =D Histology at ou.edu