Osteocyte

The most prominent cell throughout mature bone tissue is the osteocyte, an oblate-shaped form of bone cell containing dendritic processes that can survive as long as that of the organism. There are approximately 42 billion of them in an adult human body. Bone cells are also called osteocytes. Osteocytes don't really divide and have a half-life of 25 years on average. They're made up of osteoprogenitor cells, including some that go on to become active osteoblasts (that might later differentiate to osteocytes). In the mesenchyme, osteoblast, osteoclast and osteocyte are produced.

Osteocytes and related mechanisms live in lacunae (Latin for "pit") and canaliculi, respectively, throughout mature bones. Osteocytes are actually stuck osteoblasts in the matrix which they produce. They're connected by large cytoplasmic extensions which inhabit tiny canals called canaliculi, that are brought in use for nutrient and waste exchange via gap junctions.

Osteocytes meaning tell that they are actively involved mostly in regular turnover of bony matrix by different mechanosensory pathways, despite having decreased synthetic activity and (such as osteoblasts) not being supportive of mitotic division. They kill bone via a process named osteocytic osteolysis, which is quick and transient (in comparison to osteoclasts). From around the cell, calcium carbonate, hydroxyapatite, and calcium phosphate are deposited.

Site of osteocytes: A cell found inside the material of completely developed bone is called an osteoclast. It lives in a tiny chamber known as a lacuna within the calcified matrix of bone. Osteocytes are derived from osteoblasts, or bone-forming cells, and are actually osteoblasts encased in the secreted materials.

Structure of Osteocytes

Osteocytes have such a stellate form with a depth of 7 micrometres, a width of 15 micrometres, and a length of 15 micrometres The cell body has a diameter of 5-20 micrometres and contains 40-60 cellular functions each cell, with a cell-to-cell distance of approximately 20-30 micrometres. A mature osteocyte does have a single nucleus with one or two nucleoli as well as a membrane which is situated on the vascular side.

In circumferential lamellae, the cell also possess a Golgi apparatus, smaller endoplasmic reticulum, and mitochondria, as well as cell processes which radiate primarily towards a haversian canal or the bone surfaces, and outer cement line characteristic of osteons within concentric lamellar bone. Inside the mineralized collagen type I matrix, osteocytes build a vast lacuno canalicular network, including cell bodies living in lacunae and cell/dendritic processes located in canaliculi.

Development of Osteocytes

Osteocytes were found in jawless fish bones 400 to 250 million years ago, according to the fossil record. Osteocyte cell size was shown to covary through genome size; and so this relationship is being used in paleogenomic research. An osteoblast is kept behind during bone formation and becomes a "osteoid osteocyte," that retains interaction with several other osteoblasts via prolonged cellular processes.

The mechanism of osteocytogenesis is poorly understood, however the below molecules are said to play a critical role in the development of healthy osteocyte cell, in either the right numbers or even in unique distributions: dentin matrix protein 1 (DMP-1), matrix metalloproteinases (MMPs), Klotho, E11 antigen, TGF-beta inducible factor (TIEG), osteoblast/osteocyte factor 45 (OF45), oxygen and lysophosphatidic acid (LPA).

Osteoblasts distinguish into osteocytes throughout 10–20% of cases. Such osteoblasts mostly on bone surface which will be buried when osteocytes decelerate matrix development and therefore are buried by adjacent osteoblasts which are still aggressively producing matrix.

Osteocytes Function

Despite their inert nature, osteocytes function have the ability of molecular synthesis and alteration, and also signal transmission across vast distances, in a manner similar to that of the nervous system. These are by far the most popular type of bone cell (31,900 per cubic millimetre in bovine bone to 93,200 per cubic millimetre in rat bone). The mature osteocyte contains the majority of receptor activities which are essential for bone function. After a bone fracture, glutamate transporters in osteon and osteocyte release nerve growth factors, indicating the presence of a sensing and information transfer device.

The bones displayed a substantial increase in bone resorption, trabecular bone loss, decreased bone development, and failure of reaction to unloading while osteocytes were experimentally killed.

Osteocytes are mechanosensor cells which regulate the behaviour of osteoblasts and osteoclasts within a specific multicellular unit (BMU), a transient anatomic structure wherein bone remodelling takes place. Osteocytes produce an inhibitory signal which is transmitted to osteoblasts via certain cell processes, allowing them to be recruited for bone formation.

Sclerostin

Sclerostin is a secreted protein produced by osteoclasts which inhibits bone formation via attaching to LRP5/LRP6 coreceptors and suppressing Wnt signalling. Sclerostin is the very first mediator of contact amongst osteocytes, osteoblasts and osteoclasts, bone forming osteoblasts, and bone resorbing osteoclasts, and is important for bone remodelling. 

Just osteocytes produce sclerostin, which inhibits bone formation in a paracrine manner. Parathyroid hormone (PTH) and mechanical loading also inhibit sclerostin. Sclerostin inhibits the function of BMP (bone morphogenetic protein), a cytokine which triggers the formation of bone and cartilage.

Clinical Significance of Osteocytes

A clinically significant study of a gel-based in vitro 3D model for human CD34+ stem cells' osteocytic potentiality has been released. The findings show that human CD34+ stem cells have special osteogenic differentiation ability and could be used in early bone regeneration.

Senescence, apoptosis (programmed cell death), osteoclastic engulfment and/or degeneration/necrosis are all causes of osteoclast death. The number of dead osteocytes throughout bone rises with age towards less than 1 percent at birth to 75 percent over age 80. Osteocyte apoptosis is believed to be linked to a reduction in mechanotransduction, that could contribute to osteoporosis. To attract osteoclasts, apoptotic osteocytes produce apoptotic bodies that express RANKL.

In vitro, mechanical loading improves osteocyte viability and aids solute transport via the lacuno-canalicular system throughout bone, that improves nutrient and oxygen exchange as well as diffusion to osteocytes.

Skeletal unloading was shown to trigger osteocyte hypoxia in vivo, a condition in which osteocytes die and osteoclasts are recruited to resorb bone. Microdamage in bone directly correlates to osteocyte death through apoptosis, that tends to metabolise a signal to trigger osteoclasts to conduct remodelling at a weakened location.

When TGF- levels drop and the expression of osteoclast-stimulatory factors like RANKL and M-CSF rises, bone resorption is increased, resulting in net bone loss. When TGF- levels drop and the production of osteoclast-stimulatory factors like RANKL and M-CSF rises, bone resorption is increased, resulting in total bone loss.