What is the composition of a bone ?
Answers
Calcium and phosphorus is the composition of a bone.
Explanation:
The composition of a bone can be described in terms of the mineral phase, hydroxyapatite, the organic phase, which consists of collagen type I, noncollagenous proteins, other components and water. The relative proportions of these various components vary with age, site, gender, disease and treatment. Any drug therapy could change the composition of a bone. This review, however, will only address those pharmaceuticals used to treat or prevent diseases of bone: fragility fractures in particular, and the way they can alter the composition. As bone is a heterogeneous tissue, its composition must be discussed in terms of the chemical makeup, properties of its chemical constituents and their distributions in the ever-changing bone matrix. Emphasis, in this review, is placed on changes in composition as a function of age and various diseases of bone, particularly osteoporosis. It is suggested that while some of the antiosteoporotic drugs can and do modify composition, their positive effects on bone strength may be balanced by negative ones.
Introduction
Bone is a heterogeneous composite material consisting, in decreasing order, of a mineral phase, hydroxyapatite (Ca10(PO4)6(OH)2) (analogous to geologic ‘hydroxyapatite'),1 an organic phase (∼90% type I collagen, ∼5% noncollagenous proteins (NCPs), ∼2% lipids by weight)2 and water. Proteins in the extracellular matrix of bone can also be divided as follows: (a) structural proteins (collagen and fibronectin) and (b) proteins with specialized functions, such as those that (i) regulate collagen fibril diameter, (ii) serve as signaling molecules, (iii) serve as growth factors, (iv) serve as enzymes and (v) have other functions. The relative amount of each of these constituents present in a given bone varies with age,3 site,4 gender,5 ethnicity6 and health status.7 The amount, proper arrangement and characteristics of each of these components (quantity and quality) define the properties of bone. The tendency of bones to fracture depends on the quantity of mineralized tissue present (size and density) often measured by clinicians as bone mineral density or BMD8 and several other factors, grouped together as ‘bone quality'.8,9 ‘Bone quality' factors include composition (weight percent of each component), mineralization (organization of the mineral and its crystallite size and perfection), collagen content and collagen crosslinks, morphology,10 microarchitecture11 and the presence of microcracks.12 Each of these factors varies with health, disease and drug therapies. Their distribution in the heterogeneous tissue also varies with these perturbations. The focus of this review will be on the composition of bone and its site-specific variation. Materials present, their characteristics and their distribution will be discussed here. Readers are referred to the references above for more information on morphology, microarchitecture and the presence of microcracks, which will not be discussed.
Bone mineral
Hydroxyapatite is the principal component of the mineral phase of bone. This was demonstrated more than 60 years ago using X-ray diffraction, now viewed as the ‘gold standard' for such determinations.1 The quantity of mineral present in bone can be determined by a variety of techniques13 including gravimetric analyses (ash weight determination), analysis of calcium and phosphate contents, spectroscopic and densitometric analyses including bone mineral density distribution (BMDD), bone mineral density (BMD) and micro-computed tomography (micro-CT). Such methods show that the mineral content of bone ranges from ∼30%/dry weight (in the skate or ray appendicular skeletal element, the propterygium) to 98%/dry weight in the stapes of the human ear. Most bones have ∼60–70% mineral/dry weight, depending upon site, species and stage of development