Osteocyte

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Osteocyte_table_infobox_0

OsteocyteOsteocyte_header_cell_0_0_0
DetailsOsteocyte_header_cell_0_1_0
LocationOsteocyte_header_cell_0_2_0 BoneOsteocyte_cell_0_2_1
IdentifiersOsteocyte_header_cell_0_3_0
LatinOsteocyte_header_cell_0_4_0 osteocytusOsteocyte_cell_0_4_1
MeSHOsteocyte_header_cell_0_5_0 Osteocyte_cell_0_5_1
THOsteocyte_header_cell_0_6_0 Osteocyte_cell_0_6_1
FMAOsteocyte_header_cell_0_7_0 Osteocyte_cell_0_7_1

An osteocyte, an oblate shaped type of bone cell with dendritic processes, is the most commonly found cell in mature bone tissue, and can live as long as the organism itself. Osteocyte_sentence_0

The adult human body has about 42 billion of them. Osteocyte_sentence_1

Osteocytes do not divide and have an average half life of 25 years. Osteocyte_sentence_2

They are derived from osteoprogenitor cells, some of which differentiate into active osteoblasts. Osteocyte_sentence_3

Osteoblasts/osteocytes develop in mesenchyme. Osteocyte_sentence_4

In mature bones, osteocytes and their processes reside inside spaces called lacunae (Latin for a pit) and canaliculi, respectively. Osteocyte_sentence_5

Osteocytes are simply osteoblasts trapped in the matrix that they secrete. Osteocyte_sentence_6

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. Osteocyte_sentence_7

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. Osteocyte_sentence_8

They destroy bone through a rapid, transient (relative to osteoclasts) mechanism called osteocytic osteolysis. Osteocyte_sentence_9

Hydroxyapatite, calcium carbonate and calcium phosphate is deposited around the cell. Osteocyte_sentence_10

Structure Osteocyte_section_0

Osteocytes have a stellate shape, approximately 7 micrometers deep and wide by 15 micrometers in length. Osteocyte_sentence_11

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. Osteocyte_sentence_12

A mature osteocyte contains a single nucleus that is located toward the vascular side and has one or two nucleoli and a membrane. Osteocyte_sentence_13

The cell also exhibits a reduced size endoplasmic reticulum, Golgi apparatus and mitochondria, and cell processes that radiate largely towards the bone surfaces in circumferential lamellae, or towards a haversian canal and outer cement line typical of osteons in concentric lamellar bone. Osteocyte_sentence_14

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. Osteocyte_sentence_15

Development Osteocyte_section_1

The fossil record shows that osteocytes were present in bones of jawless fish 400 to 250 million years ago. Osteocyte_sentence_16

Osteocyte size has been shown to covary with genome size; and this relationship has been used in paleogenomic research. Osteocyte_sentence_17

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. Osteocyte_sentence_18

The process of osteocytogenesis is largely unknown, but the following molecules have been shown to play a crucial role in the production of healthy osteocytes, either in correct numbers or specific distributions: 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. Osteocyte_sentence_19

10–20% of osteoblasts differentiate into osteocytes. Osteocyte_sentence_20

Those osteoblasts on the bone surface that are destined for burial as osteocytes slow down matrix production, and are buried by neighboring osteoblasts that continue to produce matrix actively. Osteocyte_sentence_21

Palumbo et al. Osteocyte_sentence_22

(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). Osteocyte_sentence_23

The embedded "osteoid-osteocyte" must do two functions simultaneously: regulate mineralization and form connective dendritic processes, which requires cleavage of collagen and other matrix molecules. Osteocyte_sentence_24

The transformation from motile osteoblast to entrapped osteocyte takes about three days, and during this time, the cell produces a volume of extracellular matrix three times its own cellular volume, which results in 70% volume reduction in the mature osteocyte cell body compared to the original osteoblast volume. Osteocyte_sentence_25

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. Osteocyte_sentence_26

As the osteoblast transitions to an osteocyte, alkaline phosphatase is reduced, and casein kinase II is elevated, as is osteocalcin. Osteocyte_sentence_27

Osteocytes appear to be enriched in proteins that are resistant to hypoxia, which appears to be due to their embedded location and restricted oxygen supply. Osteocyte_sentence_28

Oxygen tension may regulate the differentiation of osteoblasts into osteocytes, and osteocyte hypoxia may play a role in disuse-mediated bone resorption. Osteocyte_sentence_29

Function Osteocyte_section_2

Although osteocytes are relatively inert cells, they are capable of molecular synthesis and modification, as well as transmission of signals over long distances, in a way similar to the nervous system. Osteocyte_sentence_30

They are the most common cell type in bone (31,900 per cubic millimeter in bovine bone to 93,200 per cubic millimeter in rat bone). Osteocyte_sentence_31

Most of the receptor activities that play an important role in bone function are present in the mature osteocyte. Osteocyte_sentence_32

Osteocytes contain glutamate transporters that produce nerve growth factors after bone fracture, which provides evidence of a sensing and information transfer system. Osteocyte_sentence_33

When osteocytes were experimentally destroyed, the bones showed a significant increase in bone resorption, decreased bone formation, trabecular bone loss, and loss of response to unloading. Osteocyte_sentence_34

Osteocytes are thought to be mechanosensor cells that control the activity of osteoblasts and osteoclasts within a basic multicellular unit (BMU), a temporary anatomic structure where bone remodeling occurs. Osteocyte_sentence_35

Osteocytes generate an inhibitory signal that is passed through their cell processes to osteoblasts for recruitment to enable bone formation. Osteocyte_sentence_36

Osteocyte specific proteins such as sclerostin have been shown to function in mineral metabolism, as well as other molecules such as PHEX, DMP-1, MEPE, and FGF-23, which are highly expressed by osteocytes and regulate phosphate and biomineralization. Osteocyte_sentence_37

The osteocyte is an important regulator of bone mass and a key endocrine regulator of phosphate metabolism. Osteocyte_sentence_38

Sclerostin Osteocyte_section_3

Osteocytes synthesize sclerostin, a secreted protein that inhibits bone formation by binding to LRP5/LRP6 coreceptors and blunting Wnt signaling. Osteocyte_sentence_39

Sclerostin, the product of the SOST gene, is the first mediator of communication between osteocytes, bone forming osteoblasts and bone resorbing osteoclasts, critical for bone remodeling. Osteocyte_sentence_40

Only osteocytes express sclerostin, which acts in a paracrine fashion to inhibit bone formation. Osteocyte_sentence_41

Sclerostin is inhibited by parathyroid hormone (PTH) and mechanical loading. Osteocyte_sentence_42

Sclerostin antagonizes the activity of BMP (bone morphogenetic protein), a cytokine that induces bone and cartilage formation. Osteocyte_sentence_43

Clinical significance Osteocyte_section_4

Clinically important research of gel based in vitro 3D model for the osteocytic potentiality of human CD34+ stem cells has been described. Osteocyte_sentence_44

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. Osteocyte_sentence_45

Osteocytes die as a consequence of senescence, degeneration/necrosis, apoptosis (programmed cell death), and/or osteoclastic engulfment. Osteocyte_sentence_46

The percentage of dead osteocytes in bone increases with age from less than 1% at birth to 75% after age 80. Osteocyte_sentence_47

Osteocyte apoptosis is thought to be related to decreased mechanotransduction, which possibly leads to the development of osteoporosis. Osteocyte_sentence_48

Apoptotic osteocytes release apoptotic bodies expressing RANKL to recruit osteoclasts. Osteocyte_sentence_49

Mechanical loading increases osteocyte viability in vitro, and contributes to solute transport through the lacuno-canalicular system in bone, which enhances oxygen and nutrient exchange and diffusion to osteocytes. Osteocyte_sentence_50

Skeletal unloading has been shown to induce osteocyte hypoxia in vivo, this is when osteocytes undergo apoptosis and recruit osteoclasts to resorb bone. Osteocyte_sentence_51

Microdamage in bone occurs as the result of repetitive events of cycling loading, and appears to be associated with osteocyte death by apoptosis, which appear to secrete a signal to target osteoclasts to perform remodeling at a damaged site. Osteocyte_sentence_52

Under normal conditions, osteocytes express high amounts of TGF-β and thus repress bone resorption, but when bone grows old, the expression levels of TGF-β decrease, and the expression of osteoclast-stimulatory factors, such as RANKL and M-CSF increases, bone resorption is then enhanced, leading to net bone loss. Osteocyte_sentence_53

Mechanical stimulation of osteocytes results in opening of hemichannels to release PGE2 and ATP, among other biochemical signaling molecules, which play a crucial role in maintaining the balance between bone formation and resorption. Osteocyte_sentence_54

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. Osteocyte_sentence_55

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. Osteocyte_sentence_56

It is now recognized that osteocytes respond in a variety of ways to the presence of implant biomaterials. Osteocyte_sentence_57

See also Osteocyte_section_5

Osteocyte_unordered_list_0


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Osteocyte.