The process of regenerating new limbs (and even
sometimes new bodies!) is very complex and only
partly understood. Basically, regeneration is the
ability of an animal's cells to make new body
parts during adulthood, just like they did during
development, most of an animal's cells take on a
particular identity--they become blood cells, lung
cells, bone cells, or whatever. This is called
differentiation, and differentiated cells almost
always keep their new identities forever. A
special type of cell called a stem cell sometimes
remains behind without a particular identity. Stem
cells can be thought of as "permanently immature"
cells that can decide later what fate to take on.
(Stem cells in your skin are what make you able to
heal after you've been cut or bruised.)
Starfish and some other animals have cells
like this that can do more than just make new
skin, they can differentiate into whole new arms.
In order for them to do this, they need to receive
the right signals from the rest of the body. If we
lose a limb, we either don't send the right
signals to our stem cells, or our stem cells are
not able to differentiate into all the parts
necessary for a whole new limb. Starfish seem to
send the right signals, and their cells are able
to differentiate properly, so they are able to
regenerate whole new limbs.
We don't know yet
exactly what those signals are, or why some stem
cells are able to differentiate more than others.
Scientists think that this may be easier for them,
because their bodies are not as complex as ours.
Starfish, if you ever look closely at one, have a
central region of the body from which the limbs
arise. If a limb is severed, a new one (small)
appears in the central region, and extends
outward. I also know that starfish limbs can
regenerate the central region (and thus all of the
other limbs) as well.
If you're asking why can
they do it but we can't, it's because of the
program of development that starfish have. Their
growth is indeterminate - they keep growing, and
all cells retain the ability to grow into whatever
proportion is needed. Our cells don't dothat. Why?
That's an evolutionary question that is still
being worked out, I expect.
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