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Starfish can regrow lost arms, and salamanders can sprout new limbs. So why can't we? Sci Show explains the science of regeneration, and explores the limitations the humans face -- and are trying to go beyond.
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Sources:
http://www.livescience.com/34513-how-salamanders-regenerate-lost-limbs.html
http://www.scientificamerican.com/article/new-limb-regeneration-ins/
http://www.sciencedaily.com/releases/2013/05/130520163727.htm?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+sciencedaily%2Ftop_news%2Ftop_science+%28ScienceDaily%3A+Top+News+--+Top+Science%29
http://www.hhmi.org/biointeractive/newt-limb-regeneration
http://www.scientificamerican.com/article/regrowing-human-limbs/
http://www.pnas.org/content/early/2013/05/17/1300290110
http://phenomena.nationalgeographic.com/2013/03/13/will-we-ever-regenerate-limbs/
http://anonymouse.org/cgi-bin/anon-www.cgi/http://phenomena.nationalgeographic.com/2012/09/26/spiny-mice-flaying-skin-healing-factor/
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Starfish can regenerate arms, certain lizards can grow back severed tails, some flatworms can recreates their entire body from a single adult cell, and my skin will grow back together after a paper cut.

On some level, we can all do a little regeneration. But if I cut my arm off, I'm not going to expect to grow a new one, right? The- the wound would event- eventually be covered over with a rugged, fibrous matrix of the protein collagen, forming what we know is scar tissue with a layer of skin on top. But I'm not going to grow a new appendix.

But that is not true for everyone. The real superstars of limb regeneration are the members of the order Caudata. Salamanders. Cut off a salamander's leg—don't actually cut off a salamander's leg, you know what I mean—and cells of the wound side won't form scar tissue. Instead, they'll transform into what researchers call a "wound epidermis," which activates a wave of chemical instructions to the cells below. Soon the nerves in the stump begin to grow again. And mature muscle and tissue cells actually revert back to their immature state. Sort of going back in time before they were specialized cells. They then start streaming toward the wound, forming a mass called the blastema.

These undifferentiated are a lot like stem cells or what embryonic cells are like during development before a gene is activated that tells them what they're going to be, like a liver or a heart or a skin cell.

But these undifferentiated cells were mature and they have a terrific memory of what they used to be, like a muscle cell in a forelimb or a cartilage cell in a leg joint. This is how they take up their specific positions and form new muscle, connective tissue, cartilage, and bone until boom! The animal has a whole shiny new leg.

We understand the basic method here, but researchers are still puzzled by the details. Like why does the wound epidermis form in the first place? And how does it trigger that reversion in the cells below it? And just how doe all those regenerating cells know where they should be and what shape they should take on? The truth is, we just don't know. Yet.

But researchers have recently pinpointed a cell that seems to be responsible for salamander's remarkable regeneration capabilities. All animals have a kind of repair cells called macrophages. They rush into a wound site and eat up dead cells and pathogens while triggering the release of other immune cells. Mammals also use them to repair muscle, which got Dr. James Godwin of the Australian Regenerative Medicine Institute to thinkin'.

When Godwin and his colleague reduced the number of macrophages at a salamander wound site, they found that regeneration took much longer. And when they removed all the macrophages, the poor guys could no longer regrow limbs, but rather ended up with a lot of scar tissue, just like we do.

This suggests that regeneration is possible because those macrophage cells release some vital chemical signal that might trigger the undifferentiated cells to come in and do their thing.

So does that mean we'll soon know how to help humans regenerate lost limbs? Don't hold your breath.

Researchers say we are still a long way off from understanding the complexity of regeneration. Plus, considering that it would take some small salamanders over a year to regenerate a limb and larger ones over a decade, even if a human could grow back a lost leg, it could take them over twenty years.

Still there are more immediate benefits and attainable goals that might come from this research.  Like how to make wounds heal faster with little, if any, scarring. Not a new arm. But still pretty awesome.

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