Osteocyte

Bone, or osseous tissue, is composed of osteocytes and osteoclasts embedded in a calcified matrix. Hard tissue consists of about 50% water and 50% solids. The solids are composed of cartilaginous material hardened with inorganic salts, such as calcium carbonate and phosphate of lime. 

Most bones of the human skeleton are composed of two structurally distinct types of tissue compact (dense) and trabecular (cancellous, spongy) bone. Both types contain the same elements cells ( osteocytes) embedded in a mineralised matrix and connected by small canals ( canaliculi ). In compact bone, which makes up 85% of the skeleton, these components form elongated cylinders of concentric lamellae surrounding a central blood vessel (called osteon or Haversian system). Cancellous bone, in contrast, forms thin,... 

Osteoblasts are the primary cells responsible for bone formation. They are derived from mesenchymal (stromal) cells that first differentiate into pre-osteoblasts and then into mature, bone matrix producing osteoblasts. Inactivated or resting osteoblasts become lining cells and thus a reservoir for bone forming cells to be activated at the next remodelling cycle. Osteoblasts trapped and embedded in the mineralised matrix are called osteocyts, and are important for many properties of living bone. 

A cell embedded within the mineralised matrix of bone. Osteocytes are derived from former osteoblasts and are responsible for intra-skeletal sensing and signalling. 

Systemic regulators of osteoblast, osteocyte and osteoclast functions, and therefore of bone metabolism. The major bone-seeking hormones are parathyroid hormone (PIH), 1,25-dihydroxy vitamin D3 (calcitriol) and the various ex hormones. 

The major cell types involved in bone resorption and deposition are osteoclasts and osteoblasts (Figure 48-11). The former are associated with resorption and the latter with deposition of bone. Osteocytes are descended from osteoblasts they also appear to be involved in maintenance of bone matrix but will not be discussed further here. 

Figure 7.2. Schematics of bone anatomy (a) the structure of a long bone demonstrating the distribution of the two different tissue structures, cortical and cancellous bone, and (b) the cells present in bone osteoblasts, bone-forming cells found on surfaces osteocytes, bone cells embedded in the mineralised matrix and osteoclasts, bone-removing cells.

Approximately 15 per cent of osteoblasts become entrapped in their own matrix to become osteocytes. Osteocytes have a vast three-dimensional network of cell processes (canaliculi), providing nourishment and cell-cell interactions. Because they are located throughout bone tissue and have an extensive canalicular network, osteocytes are assumed to be a vital component of sensing mechanical signals. Nutrients are essential for the vitality of bone tissue and are obtained from the blood supply, limiting most osteocytes to lie within 150 p.m of a blood vessel, resulting in a high cellular density 25000 osteocytes within a square millimetre of bone... 

For a skeletal element to respond to its mechanical environment, the cells in the tissue must regulate their environment in response to the mechanical stimuli they receive. The regulatory process can be thought of as a feedback loop (Figure 7.5) in which the osteocyte senses the stimulus and... 

Microscopic lesions of the proventricular epithelium, pectoral muscles, brain, proximal tubular epithelium of the kidney, and bone medullary osteocytes... 

Verborgt O, Gibson GJ, Schaffer MB (2000) Loss of osteocyte integrity in association with microdamage and bone remodeling after fatigue in vivo. J Bone Miner Res 15 60-67... 

The effects of raloxifene on bone histomorphometry were analyzed by Ott et al. (2002). In a group of 54 women enrolled in the MORE study, two transiliac bone biopsies were obtained at baseline and after 2 years of treatment. The results confirmed the safety of the drug on bone tissue since no woven bone, mineralization defect, cell toxicity, or medullary fibrosis was observed. Moreover, the number of empty osteocytic lacunae also suggested an antiapoptotic effect on the osteocyte. More recent experimental data further confirm this antiapoptotic effect of raloxifene on osteoblastic and osteocytic cells (Taranta et al. 2002). 

As they secrete osteoid matrix around themselves, osteoblasts eventually become entrapped within lacunae ( spaces ) and mature into osteocytes. Although topographically isolated from each other, osteocytes maintain contact with neighbours by cell processes which run through tiny interconnecting fluid-filled canaliculi (channels) within the matrix. 

In response to the presence of antigenic stimuli, in seasonal allergic rhinitis (hay fever), mast cells and basophils are sensitised and inflammatory mediators, such as leukotrienes and prostaglandins, are released. Osteocytes are bone cells involved in bone formation. 

Hypercalcemia, in contrast, results in calcitonin synthesis and release, while PTH release and formation of 1,25-(0H)2D2 are inhibited. Calcitonin inhibits bone resorption directly by reducing osteocyte activity. Calcitonin also induces an initial phosphate diuresis, followed by increased renal calcium, sodium, and phosphate excretion. 

Mechanism of Action Abisphosphonatethat decreases mineral release and matrix in bone and inhibits osteocytic osteolysis. Therapeutic Effect Decreases bone resorption. 

The visible evidence in matrix bone cannot be disputed. The resting osteocyte is contained within a dense mineral environment, all of which is clearly outside the membrane. It is easy to accept the idea that the matrix is laid down by the osteoblast and subsequently mineralized extracellularly. The dynamic evidence about bone formation, however, is not so clear, since a brief examination of the literature suggests that evidence is so uncertain as to require complete re-examination. The mineral at a developing bone front appears in discrete clusters or pockets with a... 

Pautard, F. G. E. Calcification in single cells with an appraisal of the relationship between Spirostomum ambiguum and the osteocyte. In The mechanisms of mineralization in the invertebrates and plants. Watabe, N., Wilbur, K. M. (eds.). The Belle W. Baruch Library in Marine Science, Vol. 5, pp. 33, The University of South Carolina Press 1976... 

The alkaline phosphatases are found in bacteria, fungi, and higher animals but not in higher plants. In E. coli alkaline phosphatase is concentrated in the peri-plasmic space. In animals it is found in the brush border of kidney cells, in cells of the intestinal mucosa, and in the osteocytes and osteoblasts of bone. It is almost absent from red blood cells, muscle, and other tissues which are not involved extensively in transport of nutrients. 

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