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 embryonic development.
During embryonic 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 do that. Why? That's an evolutionary
question that is still being worked out, I expect.
Click Here to return to the search form.