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When a person's arm is amputated, the arm will no longer be able to grow back. However, in salamanders, the arm actually grows back.

In comparison to a human, what is really happening to the cells in the wounded area in salamander ?

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Certain parts of the human will regenerate, though (liver for instance) –  nico Oct 7 '12 at 9:57
    
well it is not as fast as the salamander .. lets say an arm why wouldn't it regenerate ? –  IxariusSci Oct 7 '12 at 10:18
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Our limbs do regenerate - but only up to a certain point. If you cut off up to about 0.5cm from the tip of your finger, it will completely regenerate. (No, DO NOT try this just because I said so.) –  Armatus Oct 8 '12 at 17:25

2 Answers 2

After salamanders (or certain other amphibians) lose a limb, the epidermis migrates to cover the wound, forming a structure known as the apical epidermal cap. The underlying "stump" then generates a de-differentiated mass of cells from normal tissue. In most cases, these cells will express genes that are vital to the proper formation of limbs, such as HOX genes. This "lump" of cells at the stump of the wound are typically referred to as a blastema. Basically, the blastema continues to grow and regenerate the limb in a way similar to fetal growth. Though mammals (including humans) also have some regenerative capabilities, they are much more limited. In addition, many regenerated limbs will also be mildly deformed (square fingernails). In axolotl salamanders, it was found that certain retrotransposons, such as the LINE-1 element, were prevented from "jumping" around in the developing blastema during limb regeneration. The two active proteins PL1 and PL2 were responsible for inhibiting the activity of LINE-1 and other such retrotransposons. Active retrotransposons can potentially rearrange the genes in germ cells can be detrimental to the mechanisms that allow the blastema to replicate and differentiate, so the PL2 and PL1 proteins play a vital role in their regenerative capabilities. When the PL2 and PL1 proteins were "switched off" in certain axolotl salamanders, their regeneration was much less efficient. Since mammals lack the PL1 and PL2 proteins and have a different LINE-1 retrotransposon, they have reduced regenerative capabilities.

Sources:

Wei Zhu, Dwight Kuo, Jason Nathanson, Akira Satoh, Gerald M. Pao, Gene W. Yeo, Susan V. Bryant, S. Randal Voss, David M. Gardiner, Tony Hunter. Retrotransposon long interspersed nucleotide element-1 (LINE-1) is activated during salamander limb regeneration. Development, Growth & Differentiation, 2012; 54 (7)

Wei Zhu, Gerald M. Pao, Akira Satoh, Gillian Cummings, James R. Monaghan, Timothy T. Harkins, Susan V. Bryant, S. Randal Voss, David M. Gardiner, Tony Hunter. Activation of germline-specific genes is required for limb regeneration in the Mexican axolotl. Developmental Biology, 2012; 370 (1)

Odelberg SJ (August 2004). "Unraveling the Molecular Basis for Regenerative Cellular Plasticity". PLoS Biology 2 (8)

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For salamanders, when they lose a limb, they have cells called fibroblasts that migrate to the wounded area. These cells are not differentiated and are able to be differentiated to whatever is necessary for regeneration. That sort of cellular differentiation for repair doesn't happen for us humans, yet. Give synthetic biology about 20 more years. :D

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Sorry, but this answers only begs for the question: how are salamander fibroblasts different from human fibroblasts? By the way fibroblasts are differentiated cells. –  nico Oct 8 '12 at 9:14

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