[DoctorBeaver (OP)]

Why is it that when a bone is broken, it grows back together but when a limb is severed the bone doesn't grow? What is the mechanism that stops the bone growing & growing, trying to find the other side of the break?

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[ukmicky]

I could be wrong, and probably am but i believe for broken bones to grow and rejoin then both sides of the break have to be within a certain distance to each other, and both sides of the break then grow and rejoin.
so i assume a chemical must be released at the break informing each side of the other sides presence  which then  triggers the growth.

Michael

[Hadrian]

I would imagine the exposure to the air would trigger a new set of reactions quite different to when the brake being look after is internally. Protecting the overall integrity of the body would take over and this would prevent the bone from mending. Proximity also must play a part. To far apart and no growth will be triggered.



   


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[chris]

Great question.

When a bone breaks the two bone ends, if they are not directly opposed (touching), are usually bridged by a blood clot or thrombus. This hardens to form a scaffold across which bone-forming cells can migrate from the two severed bone-ends. As the cells flock in they communicate with each other chemically and physically, defining their territories and assigning the repair work.

Initially they form a very rough approximation of the repair - a holding measure rather like wrapping an exhaust bandage around a broken car exhaust. This is a callus. Then, over time, the repair is slowly re-modelled, by removing and re-building new bone, to restore the bone to normal structure.

When a bone is cut with a saw, as in an amputation, there is a degree of localised bleeding, and the cut stump will remodel itself, but local pressure effects help to confine and constrain the behaviour of the cells, otherwise the entire stump would turn to bone, and Captain Ahab would have had no use for his prosthetic leg made from a whale bone.

The bottom line is that the scaffold for regeneration is what is critical because cells need guidance as to where to go and where to grow. For instance, if you damage the surface of your skin it can repair. But destroy the full thickness of the skin, removing the scaffold, and there's no guide for regeneration. That's why, for example, you don't regrow a foreskin after a circumcision.


Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[DoctorBeaver (OP)]

Thanks for that, Chris... very nicely explained.

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