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Bone is living tissue, rebuilt throughout life. Cells called osteoclasts dissolve small patches of old bone and cells called osteoblasts fill them with new bone, renewing roughly a tenth of an adult skeleton each year.

Human biology

HealthScience

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In the first year of life almost the entire skeleton is replaced; in adults the turnover slows to about 10% a year. This constant remodelling repairs tiny cracks from everyday stress, stores or releases calcium and phosphate as the body needs them, and responds to load, which is why bones that bear more weight stay denser. The balance matters: when breakdown outpaces building, as often happens after menopause, bones thin and osteoporosis can follow. Astronauts in weightlessness lose bone quickly because their skeletons carry no load. Different bones and regions renew at different rates, so the popular claim that you get a 'new skeleton every ten years' is too neat: some bone is renewed quickly, while other parts last for decades.

Türkçe oku
Kemik, ömür boyu yenilenen canlı bir dokudur. Osteoklast denen hücreler eski kemikten küçük parçaları eritir, osteoblast denen hücreler de boşluğu yeni kemikle doldurur; böylece yetişkin bir iskeletin yaklaşık onda biri her yıl yenilenir.

Yaşamın ilk yılında iskeletin neredeyse tamamı yenilenir; yetişkinlikte bu hız yılda yaklaşık %10'a düşer. Bu sürekli yenilenme, gündelik yüklerin açtığı küçük çatlakları onarır, vücut ihtiyaç duydukça kalsiyum ve fosforu depolar ya da serbest bırakır ve kemiğe binen yüke göre ayarlanır; daha çok ağırlık taşıyan kemiklerin daha yoğun kalmasının nedeni budur. Denge önemlidir: Yıkım yapımı geçtiğinde, örneğin menopozdan sonra sık görüldüğü gibi, kemikler incelir ve kemik erimesi (osteoporoz) başlayabilir. Ağırlıksız ortamda kalan astronotlar da iskeletleri yük taşımadığı için hızla kemik kaybeder. Kemikler ve bölgeler farklı hızlarda yenilendiği için “iskelet her on yılda baştan yenilenir” sözü fazla düzgün bir basitleştirmedir: Bazı kısımlar hızla yenilenirken bazıları on yıllarca yerinde kalır.

Sources and further reading

Background reading: Bone remodeling
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