Infant bone fractures heal without any medical intervention, thanks to muscle contractions and tissue growth that together move the bone fragments back into place.
Elazar Zelzer and his colleagues at the Weizmann Institute of Science in Rehovot, Israel, imaged fractured bones as they healed in newborn mice. They found that a soft callus (pictured in purple) containing cartilage forms at the fracture site and acts like a mechanical jack: the growth of tissue (red arrows) on one side of the fracture provides force to realign the bone fragments. The callus hardens (orange), allowing the bone to heal.
Botulinum toxin, which paralyses muscles, blocked the repair process in the mice, suggesting that muscle contractions are also required for infant bone healing.
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How baby bones self-repair. Nature 515, 10 (2014). https://doi.org/10.1038/515010c