The skeleton provides scaffolding which enables a mobile terrestrial life. The skeleton consists of bones, joints, tendon sheaths and fascia. The bony skeleton forms the firm attachment for tendons on which muscles pull for movements. Bones consist of 35% mineral salts (largely calcium and phosphorus), 20% organic matter (mostly collagen) and 45% water. 99% of the body calcium and two thirds of the body magnesium is found in bone.
• The vertebrae support the whole body.
• The long bones of the limbs, humerus, radius, ulna, tibia, fibula and femur, support muscles of movement and give height to the individual; their shape is designed for an upright stance.
Every bone will have some role in each of these functions but some will be more conspicuous in a particular function than others:
Almost all bones have joints. In the long bones these are at either end. The bones of the wrist and ankle are small and are largely articular surfaces. Some bones have only one articular surface, e.g. the scapula. The protective function of the bones is best illustrated by the pelvis, skull, vertebrae and sternum and ribs. The pelvic bones protect the ovaries, uterus, bladder and rectosigmoid region of the colon. The skull and vertebrae protect the brain and spinal column and the sternum and ribs, the heart and lungs. Each bone has a blood supply, which has a nutrient role A long bone (Figure 41.2) consists of:
• a shaft or diaphysis
• an epiphysis, which is the end of the bone and includes the articular surface
• a metaphysis, between the diaphysis and the epiphysis, a functional zone
• an epiphyseal plate, cartilage between the epiphysis and the metaphysis; which is, important in growing bone
1. The skeleton consists of bones, joints, tendon sheaths and fascia. The bony skeleton forms the attachment for tendons allowing muscles to contract. Bones consist of 35% mineral salts (largely calcium and phosphorus), 20% organic matter (mostly collagen) and 45% water; 99% of the body calcium is found in bone.
2. Each bone has a blood supply, which provides nutrients. A long bone consists of a shaft or diaphysis; an epiphysis with an articular surface; a metaphysis; a functional zone and an epiphyseal plate; cartilage between the epiphysis and the metaphysis is important in growing bone.
3. Bone is formed in three layers, periosteum, cortex and medulla. Periosteum is a fibrous membrane covering the bone. The cortex is the hard sheath of compact bone. The medulla contains the marrow and spongy (cancellous) bone.
4. There are three types of bone cells. Osteoblasts line the external surface of the bone trabeculae, the inner aspect of the periosteum and the surface of bone lining the osteons; they produce collagen and proteins forming the organic bone matrix, and deposit and remove calcium. Osteocytes are osteoblasts trapped in mature bone; they have a role in mature bone maintenance and are the sensors for the changes in response to mechanical demand. Osteoclasts are responsible for bone resorption.
5. Bone consists of lamellae and a mineral matrix of calcium hydroxyapatites deposited on a protein collagen matrix, which gives bone rigidity. The organic lamellae is 90% collagen (largely type I) produced by the osteoblast and calcification occurs between the collagen molecules. Bone contains other proteins, phosphoproteins and lipids.
6. Bone is continuously remodelled throughout life. The rate varies with age. Bones exposed to stress, e.g. weight-bearing and exercise, will remodel at a different rate from less-pressured bones.
7. With age, bone develops osteoporotic changes wherein bone structure and density decline with a continued calcium content.
8. Bone calcium content declines in osteomalacia.
Further Reading —
Alliston T, Derynck ( 2002) Interfering with bone remodelling Nature 416, 686-7
Anderson, D. (1992) Osteoporosis in men. British Medical Journal, 305, 489–90.
Anderson, J.J.B. (1992) The role of nutrition in the functioning of skeletal tissue. Nutrition Review, 50, 388–94.
Beattie, J.H. and Avenell, A. (1992) Trace element nutrition and bone metabolism. Nutrition Research Reviews, 5, 167–88.
Branca, F, Robins, S.P., Ferro-Luzzi, A. and Golden, M.H.N. (1992) Bone turnover in malnourished children. Lancet, 340, 1493.
Browner, W .S., Seeley, D.G., Vogt, T .M. and Cummings, S.R. (1991) Non-trauma mortality in elderly women with low bone density. Lancet, 338, 355–8.
Cullen, K. (1992) Motivating people to attend screening for osteoporosis.British Medical Journal, 305, 521–2.
Cummings, S.R., Black, D.M., Nevitt, M.C., Browner, W, Cauley, J., Ensrud, K., Genant, H.K., Palermo, L., Scott, J. and Vogt, T.M. (1993) Bone density at various sites for prediction of hip fractures. Lancet, 341, 72–5.
Cummings SR, Helton LJ III ( 2002 ) epidemiology and outcome of osteoporotic fractures. Lancet, 359, 1761-67.
Dawson-Hughes B, Harris SS, Krall EA, Dallal GE ( 1997) Effect of calcium and vitamin D supplementation on bone density in men and women 65 years of age or older . New England Journal of Medicine. 337, 670-676
Delmas PD ( 2001) Osteoporosis in patients with organ transplants: a neglected problem. Lancet 357, 325
Dempster, D.W and Lindsay, R. (1993) Pathogenesis of osteoporosis.Lancet, 341, 797–801.
Johnston, C.C. and Slemenda, C.W (1992) Changes in skeletal tissue during the ageing process. Nutrition Reviews, 50, 385–7.
Keay N (2000) The modifiable factors affecting bone mineral accumulation in girls: the paradoxical effect of exercise on bone. British Nutrition Foundation Nutrition Bulletin 25 219-222.
Kun Z, Greenfield H, Xueqin D, Fraser DR ( 2001) Improvement of bone health in childhood and adolescence Nutrition Research Review. 14, 119-151
Leader (1992) Maximizing peak bone mass: calcium supplementation increases bone mineral density in children. Nutrition Reviews, 50, 335–7.
Lees, B., Molleson, T, Arnett, R.A. and Stevenson, J.C. (1993) Differences in proximal femur bone density over two centuries. Lancet, 341, 673–5.
Loveridge, N., Farquharson, C. and Scheven, B.A.A. (1990) Endogenous mediators of growth. Proceedings of the Nutrition Society, 49, 443–50.
Namgung R, Tsang RC ( 2000) Factors affecting newborn bone mineral content: in utero effects on newborn bone mineralisation. Proceedings of the Nutrition Society, 59, 55-63.
New SA (2001) Exercise, bone and nutrition. Proceedings of the Nutrition Society 60, 265-274.
Prentice A ( 2001) The relative contribution of diet and genotype to bone development. Proceedings of Nutrition Society. 60, 45-52
Reeve, J., Green, J.R., Hesp, R. et al. (1990) Determinants of axial bone loss in the early post menopause: The Harrow postmenopausal bone loss study, in Osteoporosis 1990 (eds C. Christiansen and K. Overgaard), Osteopress, Copenhagen, pp. 101–3.
Schneider, V .S., LeBlanc, A. and Rambaut, P .C. (1989) Bone and mineral metabolism, in Space Physiology and Medicine (eds A.E. Nicogossian, C.L. Huntoon and S.L. Pool), Lea & Febiger, Philadelphia, pp. 214–21.
Seeman E ( 2002) Pathogenesis of bone fragility in women and men. Lancet ,359, 1841-50.
Symposium on :Nutritional aspects of bone. (1997) Proceedings Nutrition Society 56, 903-914.
Weaver CM ( 2000) The growing years and prevention of osteoporosis in later life. Proceedings of the Nutrition Society. 59, 303-306
Weatherall, D.J., Ledingham, J.G.G. and Warren, D.A. (1996) Oxford Textbook of Medicine, 3rd edn, Oxford University Press, Oxford.
Wosje KS, Specker BL ( 2000) Role of calcium in bone health during childhood Nutrition Reviews 58, 253-268.