A Staff Report from the Straight Dope Science Advisory Board

Why can't we regenerate limbs like other species?

July 19, 1999

Dear Straight Dope:

Why can't human beings regenerate limbs?  What mechanism enables other animals to do so?

It's the price you pay for your more complex cellular organization.  The downside is that if you get an arm cut off, you can't regrow it. But the bright side is you don't have to live your life in a mud flat eating plankton.

The issue here isn't so much what other organisms have that we don't, but rather what we have LOST that just about everything else has always had. The capacity for regeneration is a matter of cellular recognition during development. A good parallel is the comparison between, say, a pack of wolves and a termite colony. In a pack of wolves, one's identity and relationship to other pack members are flexible things.  You can be the alpha male one day, and kicked out of the pack the next (though a male wolf will not suddenly turn into a female wolf, so there ARE limits). In a termite, once you reach adulthood as a queen/king, you are STUCK.

This same sort of interaction takes place in most organisms at the cellular level. In many organisms, cellular identity has a certain degree of flexibility, and what a given cell turns into depends on what its neighbors are. But there are often restrictions built into this, tending to become more restrictive as an organism becomes more complex. So, just as a termite MAY be able to switch from worker to queen if the switch occurs before adulthood, but loses that ability later, some organisms lose the capacity to regenerate once they're past a certainly developmental stage. Humans fall into that category. If a newborn baby loses a fingertip, it will regenerate; if a 30-year-old loses a fingertip, that's that.

What it comes down to is that when an organism is wounded, depending on which type of tissue is involved at the wound site, the repair mechanism may or may not involve cells that still have flexibility in their "programming." If the cells are still flexible, then they act like they might in an embryo; check what your neighbor cells are, and adjust accordingly. It's like having a big blueprint that only tells you what goes next to what--you can rebuild a limb or organ that way as long as the cells have some sort of orientation info available (which way is up, where the head/tail is, etc.).   Such cells will keep budding off new cells until the plan in the blueprint is completed. If the cells are NOT flexible, as in most adult vertebrates, then when the repair occurs, the damage is filled in with tissue lacking much identity, and which doesn't produce new cells: scar tissue.

To finish the story, some vertebrates can regenerate portions of their anatomy after reaching adulthood, most notably amphibians (salamanders especially; that they should retain such abilities is not surprising when you consider that amphibians undergo metamorphosis, so their cells HAVE TO retain flexibility), and some snakes and lizards that can regrow tails. Not us humans. If we have cells that lose their identity and start to proliferate, we call them "cancer."

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