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Bone is living tissue. In adults about a tenth of the skeleton is broken down and rebuilt each year, so most of it is renewed over roughly ten years; in a baby's first year, almost all of it is.

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The work is done by two kinds of cells. Osteoclasts dissolve small patches of old or damaged bone, and osteoblasts then fill the gap with new bone, which hardens as calcium and phosphate are laid down in it. This cycle, called remodelling, repairs the tiny cracks caused by everyday movement and lets bone adapt to the loads it carries: in the 19th century the German surgeon Julius Wolff described how bone grows stronger where it is stressed. Bone is also the body's calcium store: bones and teeth hold about 99 per cent of it. In space, without gravity loading the skeleton, astronauts lose more than 1 per cent of their bone mass a month. From middle age, breakdown slowly outpaces rebuilding, which is why osteoporosis, the thinning of bone, becomes more common with age.

Türkçe oku
Kemik canlı bir dokudur. Yetişkinlerde iskeletin her yıl yaklaşık onda biri yıkılıp yeniden yapılır; böylece on yıl kadar içinde büyük kısmı yenilenir. Bebeğin ilk yılında ise iskeletin neredeyse tamamı yenilenir.

Bu işi iki tür hücre üstlenir. Osteoklastlar eski ya da hasarlı kemikten küçük parçaları eritir, osteoblastlar da açılan boşluğu yeni kemikle doldurur; yeni kemik, içine kalsiyum ve fosfat yerleştikçe sertleşir. Kemiğin yeniden yapılanması denen bu döngü, günlük hareketlerin açtığı küçük çatlakları onarır ve kemiğin taşıdığı yüke uyum sağlamasını mümkün kılar. Alman cerrah Julius Wolff 19. yüzyılda, kemiğin yük bindiği yerde güçlendiğini ortaya koymuştu. Kemik aynı zamanda vücudun kalsiyum deposudur: Vücuttaki kalsiyumun yaklaşık yüzde 99'u kemiklerde ve dişlerde bulunur. Yer çekiminin iskelete yük bindirmediği uzayda astronotlar her ay kemik kütlelerinin yüzde 1'inden fazlasını kaybeder. Orta yaştan itibaren yıkım, yapımı yavaş yavaş geride bırakır; kemik erimesinin (osteoporoz) yaşla sıklaşmasının nedeni budur.

Sources and further reading

Background reading: Bone remodeling
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