Mineralization bone tissue

Bone cells and products collagen. With the advent of high-resolution transmission and scanning electron microscopy, the intimate relationships between the cells (Figures 25 (a)-(c)) that form (osteoblasts), maintain (osteocytes), and remodel (osteoclasts) mineralized bone tissues have been depicted and their products and reactions studied. The first extracellular products of the osteoblast are bioorganic molecules dominated by the fibrous asymmetric protein... 

As mentioned before, osteoclasts re-mineralize bone tissue and produce ions that will be reused for new bone formation, or as eofactors in many biologieal proeesses. HA hydrolyzes in many steps as shown in Figure 7.18. This process is governed by the pH of the surroimding environment, and this is one reason why drinking aeidie drinks, like soda, may lead to the erosion of tooth enamel. 

Bone is a porous tissue composite material containing a fluid phase, a calcified bone mineral, hydroxyapatite (HA), and organic components (mainly, collagen type). The variety of cellular and noncellular components consist of approximately 69% organic and 22% inorganic material and 9% water. The principal constiments of bone tissue are calcium (Ca ), phosphate (PO ), and hydroxyl (OH ) ions and calcium carbonate. There are smaller quantities of sodium, magnesium, and fluoride. The major compound, HA, has the formula Caio(P04)g(OH)2 in its unit cell. The porosity of bone includes membrane-lined capillary blood vessels, which function to transport nutrients and ions in bone, canaliculi, and the lacunae occupied in vivo by bone cells (osteoblasts), and the micropores present in the matrix. 

Under normal circumstances, the skeleton undergoes a dynamic process of bone remodeling. Bone tissue responds to stress and injury through continuous replacement and repair. This process is completed by the basic multicellular unit, which includes both osteoblasts and osteoclasts. Osteoclasts are involved with resorption or breakdown of bone and continuously create microscopic cavities in bone tissue. Osteoblasts are involved in bone formation and continuously mineralize new bone in the cavities created by osteoclasts. Until peak bone mass is achieved between the ages of 25 and 35, bone formation exceeds bone resorption for an overall increase in bone mass. Trabecular bone is more susceptible to bone remodeling in part owing to its larger surface area. 

Bone is an extremely dense connective tissue that, in various shapes, constitutes the skeleton. Although it is one of the hardest structures in the body, bone maintains a degree of elasticity owing to its structure and composition. It possesses a hierarchical structure and, as most of the tissues, is nanostructured in fact, it is a nanoscaled composite of collagen (organic extracellular matrix) and hydroxycarbonate apatite, (HCA, bone mineral). This nanostructure is in intimate contact with the bone cells (several microns in size), which result (at the macroscopic level) in the bone tissue. Figure 12.2 shows the bone hierarchical ordering from the bone to the crystalline structure of HCA. 

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). 

It appears from these studies that the postprandial responses of postmenopausal women may differ from those of younger men. In studies in which calciuria has been demonstrated, the detrimental effect of such a condition on bone has been implied. Osteoporosis is defined as a diminution of skeletal mass in which the bone is normally mineralized but the amount of bone tissue is lower than normal. Unfortunately, it is seldom diagnosed unless bone fracture occurs 30% of skeletal mass may have been lost by this time (23). Osteoporosis occurs more frequently in females then males and has been estimated as the cause of 90% of all fractures in people over the age of 60 yr (28). Therefore, differences in the postprandial responses of postmenopausal women, a population at high risk for osteoporosis, and men may help to identify contributing factors to the etiology of bone loss. 

Osteoporosis is a metabolic bone disease characterized by low bone mass and micro-architectural deterioration of bone tissue. This will lead to bone fragility and consequent increase in bone fracture risk. Mean bone mineral density (BMD) is measured with dual X-ray absorptiometry (DEXA) and expressed in Tsc (Tscore). WHO standards are a Tsc that is 1 standard deviation (SD) below mean BMD is graded as normal bone, Tsc between 1 and 1.5 SD below mean BMD is graded as osteopenia and a Tsc of more than 2.5 SD below mean BMD is graded as osteoporosis. When the Tsc is below 1.5 SD mean BMD prevention of osteoporosis must be initiated. Primary osteoporosis is caused mainly by hormone deflciency in both women and men. Secondary osteoporosis may result from endocrine, metabolic, nutritional and autoimmune causes or from immobility because of trauma. Also the use of medicaments such as corticosteroids may be contributing. 

A variety of diseases can affect bone and its structure. Paget s disease, for example, is a disorder arising from abnormal osteoclasts, characterized by exeessive bone resorption followed by replacement of the normal mineralized bone with structurally weak, poorly mineralized tissue. However, the most important bone disease is osteoporosis. This is a skeletal bone disease characterized hy microarchitectural deterioration of bony tissue and loss of bone mass, yielding increased susceptibility to bone fracture and bone fragility. In the United States, osteoporosis results in 1.5 million hone fractures annually, with 250,000 of these being hip fractures that sometimes ultimately culminate in patient death. There is a variety of therapies for the prevention and treatment of osteoporosis. 

FIGURE 2-21 The pH optima of some enzymes. Pepsin is a digestive enzyme secreted into gastric juice trypsin, a digestive enzyme that acts in the small intestine alkaline phosphatase of bone tissue, a hydrolytic enzyme thought to aid in bone mineralization. 

Hard bone tissue contains Ca, P04, OH, small amounts of carbonate, magnesium and sodium and trace elements of fluorine, chlorine and sulphur. That is why the Ca/P value in bone is not 1.67. Substitution of strange ions results in a change in the crystal structure. Consequently it is impossible to imitate the mineral part of bone exactly. 

An important caution is that any data reduction procedure that begins with extraction of principal components is based on the assumption that the data set consists of a small number of constituents that contribute to the measured spectrum at any point in proportion to their composition at that point. These methods cannot be used if that assumption is not correct. For example, mechanical loading of bone tissue causes wave number and intensity shifts in both mineral and matrix bands. While the shifts are not always linear with Appl. load, they do vary continuously. 

The first subsurface bone tissue Raman spectroscopic measurements were performed using picosecond time-resolved Raman spectroscopy on excised equine cortical bone [56, 57], In these experiments it was shown that a polystyrene backing could be detected through 0.3 mm of bone. The same picosecond technology was used to perform the first transcutaneous Raman spectroscopic measurements of bone tissue [58]. In this study, the cortical bone mineral/matrix ratios of excised limbs of wild type and transgenic (oim/oim) mice were compared and the differences demonstrated. 

Transcutaneous Raman spectroscopic measurements using spatially offset optical fibers were reported less than a year later [59, 60]. The test systems were chicken tibiae and the humeri of human cadavers. The use of cadaveric and ex vivo specimens allowed validation of the measurements by comparison to exposed bone tissue. In these measurements a depth of 3-4 mm below the skin was reached. In vivo measurements began with a report of the Raman spectrum of a phalange of a human volunteer [61]. The periosteal surface was probably 1-2 mm below the skin and the mineral phosphate Vi was accurately reproduced, although incomplete separation of mineral and matrix spectra introduced errors in other bands. 

Raman spectroscopy can be used for live, in situ, temporal studies on the development of bone-like mineral (bone nodules) in vitro in response to a variety of biomaterials/scaffolds, growth factors, hormones, environmental conditions (e.g. oxygen pressure, substrate stiffness) and from a variety of cell sources (e.g. stem cells, FOBs or adult osteoblasts). Furthermore, Raman spectroscopy enables a detailed biochemical comparison between the TE bone-like nodules formed and native bone tissue. Bone formation by osteoblasts (OB) is a dynamic process, involving the differentiation of progenitor cells, ECM production, mineralisation and subsequent tissue remodelling. 

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