Why can’t humans regenerate body parts? We’ve got the genes

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This acorn worm has grown a new head, internal organs and neural tube within fifteen days after being cut in half. Scientists believe humans have nearly all the genes needed to regenerate in a similar way, if the process is unlocked(Credit: Shawn Luttrell/University of Washington)

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Some of our closest invertebrate cousins, like this Acorn worm, have the ability to perfectly regenerate any part of their body that's cut off - including the head and nervous system. Humans have most of the same genes, so scientists are trying to work out whether human regeneration is possible, too.

Regeneration – now that'd be a nice superpower to have. Injure an arm? Chop it off and wait for it to grow back. Dicky knee? Ingrown toenail? Lop off your leg and get two for one!

It sounds ridiculous, but there's a growing number of scientists that believe body part regeneration is not only possible, but achievable in humans. After all, not only are there plenty of animals that can do it, we can do it ourselves for our skin, nails, and bits of other organs.

What's more, we've got a lot of the genes for it. "I really think we as humans have the potential to regenerate, but something isn't allowing that to happen," says Billie Swalla, director of Friday Harbor Laboratories and and a Biology professor at the University of Washington, and part of a team that's closely studying regeneration in some of our invertebrate relatives. "I believe humans have these same genes, and if we can figure out how to turn on these genes, we can regenerate."

Swalla and research partner Shawn Luttrell, also from the University of Washington, have been looking at the acorn worm; a small aquatic worm that burrows in the sand around coral reefs.

Acorn worms are interesting for two reasons. Firstly, they have the ability to regenerate every part of their body, including the head, nervous system and internal organs. Cut one in half, and within 15 days each half will regenerate into a whole worm so perfectly you couldn't distinguish it from one that had never been cut.

But secondly, they're also remarkably similar to humans, both genetically and in terms of how their body structure is laid out. In fact, thanks to their ancestral relationship with chordates like ourselves, acorn worms have a lot of DNA in common with us.

"We share thousands of genes with these animals, and we have many, if not all, of the same genes they are using to regenerate their body structures," says Luttrell, "This could have implications for central nervous system regeneration in humans if we can figure out the mechanism the worms use to regenerate."

Through DNA, every cell in our bodies contains the roadmap to build or re-build the entire machine. But for some evolutionary reason, this process has been blocked off. Perhaps we're flat out too big for it to be worthwhile from an energy perspective, as opposed to smaller amphibians and fish. Maybe our immune systems spoil the party by building up scar tissue around cuts.

Thus, the researchers have been trying to figure out the gene expression patterns that happen when these Acorn worms are regenerating. They suspect there's some sort of "master control" gene that starts the process off, because once it begins, it follows the same steps in every worm.

They're also trying to work out exactly which types of cells the worms use as the building blocks of a regeneration – be they stem cells, or other cells that could be repurposed for regrowth.

The eventual goal is to learn how to activate the process in other animals, including humans, through gene editing or activation, and supply the necessary materials to let it work.

It's a complex problem, but genetically we're working from a strong starting point. And if it's possible to regenerate tissue the same way as an Acorn worm does, that will include the nervous system, heart and other internal organs. A pretty amazing process to think about, but could this be an accepted medical reality in 100 years?

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