We think of bones as being the most stable part of the body that may still lie somewhere thousands of years from now, exciting future archaeologists. Despite their durability, bones are shaped throughout life and not only during growth or after injury. Changes in the load placed on them, as muscles get stronger or weaker, as bodies get heavier or lighter, will lead to remodelling. Bones are also the main site where blood cells are born and their insides need to be sculpted to make space for this. Our skeleton is in a constant state of remodelling, controlled by a balance between bone deposition and resorption. The cells that lay down bone are called osteoblasts while their counterparts that dissolve it are osteoclasts, a type of macrophage. If this balance is upset, for example by metabolic, dietary or hormonal changes, various bone diseases such as osteoporosis – ‘brittle bones’ – result.

Bone is made from a protein scaffold, mostly comprised of collagen, overlaid with minerals which makes them strong yet lightweight and hard yet shock absorbent. To remodel bone, osteoclasts secrete acid to dissolve the mineral and enzymes to digest the collagen protein. These are produced from long, finger-like processes on one side of the cell, the ruffled border, which increases the surface area available for resorption. The rest of the cell has a foamy appearance due to intracellular vesicles which are responsible for transporting and recycling the breakdown products.

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