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The human body can repair skin/organ laceration, fractures, even repair nerves - albeit the duration and rate of recovery differ. For instance:

The burn scar on my arm from the hot soldering iron was smooth earlier, now even the scar has grown hair over it. My rib, having suffered a nasty contusion 7 months ago, still troubles me on occasion almost to the point I must stop aught all. A school-mate who suffered a compound fracture of the tibia is recovered to the point he walks again; albeit with a slight limp.

Why can the heart muscles not repair themselves?

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I think you may need to specify to what degree of damage... increased muscle strength is (in part) achieved by damage to the old tissue, which is then repaired by immune and stem cells. At least, that's how skeletal muscle works. This differs massively from the kind of damage that impairs the function of that muscle/bone/whatever (e.g. a break/fracture). You can rest and recover from a broken bone. You die from a 'broken' heart (excuse the expression). I will be very interested to see an in depth answer. –  Luke Jul 18 '12 at 11:39
    
Erm. I don't have an answer )+: just lots of inane questions –  Everyone Jul 18 '12 at 11:48
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Heart muscles aren't the only exception: think of neurons. Once they're dead, they're gone for good. –  LanceLafontaine Jul 18 '12 at 15:03
    
@LanceLafontaine: Wasn't there an article online that said neurons regenerate - albeit at an extremely slow rate. The magnitude of regeneration was something like 1mm , but i forget the unit of time over which this growth was determined. –  Everyone Jul 18 '12 at 18:32

2 Answers 2

Part of the answer may be(*) that in bone, you have still stem cells everywhere (blood capillaries that grow until checked and osteoblasts), i.e., also in the vicinity of the wound. The same applies to skin, by the way, where the epidermis continues to produce skin layers until eternity. This facilitates repair enormously, as new cells won't have to move far to take their place.

Contrast this with heart or spinal cord, where the placement of the cells took place in early development when distances were small and most cells were undifferentiated. Also, no tissue like epidermis was left in the vicinity that could act as a near source of new cells.

(*) I said "may be" because 1. I have no ref for this, and 2. I have no idea to what extent other factors are involved. I would be thankful for counterexamples.

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+1 for the suggestion. At the same time I must beg to differ to the extent that some repair happens in the eye - an organ which develops early; why should the heart then be different? –  Everyone Jul 18 '12 at 18:40
    
thanks for the counterexample –  rwst Jul 19 '12 at 6:39

The heart does have stem cells in it, and there is cell turnover in the heart, of about 1% per year. Which is much slower than your skin, but not nothing. This allows your heart to grow during your life, and remodel itself slightly to become stronger/more efficient when you get in shape.

The heart can repair itself, when damaged it doesn't simply stay damaged. Unfortunately, the 'repair' leaves particularly useless scar tissue. After a heart attack, the dead muscle does repair itself, but very poorly. This 'scar' barely contracts, and isn't as strong as the heart wall around it.

This is mostly a function of the very specialized heart myocytes, and the evolutionary (relative) uselessness of being able to regenerate your heart after injury. In the wild, if your heart was injured, you were probably dead.

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