Figure 5. Three-dimensional array of collagen molecules. The rod-shaped molecules lie in a staggered arrangement which acts as a template for bone mineralisation. Bone mineral is laid down in the gaps.
Components of bone
The main components of bone are:
Protein matrix of collagen fibres
Collagen is a family of fibrous proteins which have the ability to form insoluble fibres of remarkable strength. The main collagen in bone is type I. The fibres are formed from a staggered array of rod-shaped units (Figure 5). Each rod consists of three polypeptide chains wrapped round each other. The chains themselves are helices.
The mineral component of bone is basically an inorganic calcium compound called hydroxyapatite, made up of calcium ions, phosphate ions and hydroxyl ions in the ratio Ca5(PO4)3(OH).
In bone some of the phosphate ions are replaced by carbonate ions. The bone mineral forms long fine crystals which add strength and rigidity to the collagen fibres. The process by which it is laid down is called mineralisation.
Figure 6. A section of bone showing osteoblasts and osteoclasts.
Three types of cell are involved in bone turnover:
Osteoblasts are responsible for building new bone and lie at the centre of bone physiology. Their functions include the synthesis of collagen and the control of mineralisation.
Osteoclasts are specialised cells that resorb bone. They work by sealing off an area of bone surface then, when activated, they pump out hydrogen ions to produce a very acid environment, which dissolves the hydroxyapatite.
Bone adapts to applied forces by growing stronger in order to withstand them; it is known that exercise can help to improve bone strength. Osteocytes are thought to be part of the cellular feed-back mechanism which directs bone to form in the places where it is most needed. They lie within mineralised bone and it is thought that they may detect mechanical deformation and mediate the response of the osteoblasts.
Figure 7. An osteoclast cell attached to the bone surface. Where it is in contact with the bone it forms a ruffled zone under which resorption takes place.
The turnover of bone mineral
The skeleton is constantly being resorbed and rebuilt by teams of bone cells in a process known as remodelling. Cell by cell a childs skeleton is replaced every 2 years, an adults every 7 10 years. The balance in this dynamic process shifts as people grow older; in youth it favours the formation of bone but in old age it favours resorption. The cellular system around which the process is centred is the bone multicellular unit (BMU). The cycle, which begins with the activation of bone resorption and ends with the rebuilding of bone, is shown in Figure 8. One of these cycles may take up to 6 months. The reason why it begins is not yet fully understood.
There are millions of BMUs on the surface of trabecular bone and within cortical bone, all at different stages of the cycle.