One of the main reasons why Doctor Who – both the show and the character – has survived so long is the concept of regeneration. Every time the Doctor dies, the character is renewed – their cells rewritten, their wounds repaired, their severed limbs regrown.
It’s a bittersweet process and one that Jodie Whittaker’s 13th Doctor has just undergone, when her final episode aired Sunday 23 October 2022.
The idea of regeneration so extreme that it can transform one person into another is, of course, a bit fanciful. But the basic tenets of regeneration (extending life, regrowing body parts) is science fact in the animal kingdom – and one day, it might be for us too.
“I’m old enough to say, ‘never say never,’” says Alejandro Sánchez Alvarado, a molecular biologist at the Stowers Institute for Medical Research in Kansas. Alvarado studies the regenerative abilities of planarian flatworms. Remember that episode of Doctor Who where a new David Tennant grows out of the 10th Doctor’s severed hand? Well, flatworms do that for fun. “They can be sliced and diced in every imaginable way,” says Alvarado, “and each piece will regenerate a complete animal.”
There are other examples too. Deer can regrow their antlers exactly as they looked before. The spiny mouse can have its skin torn off by a bird and grow it back. Salamanders and newts can repair limbs and organs. Snails can lose an eye nearly as complex as a human’s and regenerate a new one. Some species of Hemichordate, a group of marine invertebrates, can spring a new head after decapitation.
These animals share some factors. They tend not to scar over when injured, meaning the wound epidermis can remain in contact with the underlying tissue. They tend to have an abundance of stem cells and/or cells that display some degree of plasticity. But what leads to certain animals developing those attributes? How do they use them to regenerate? And why can certain animals regenerate limbs and humans can’t? “Those are the kinds of questions that we’re struggling with,” says Alvarado.
Humans do have some regenerative abilities. The liver can repair itself. Our blood system is constantly regenerating. And there are cases of people regrowing the tops of severed fingertips (if the wound is left open). “You have to lose your digit without losing the base of your fingernail,” says Alvarado. “The base has stem cells that can get activated to produce not just the nail but also connective and bone tissue.”
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Such regenerative quirks suggest that humans once had the ability to regenerate, but it’s now dormant. “The question is,” asks Alvarado, “if we had it, why did we lose it?” Theories range from regeneration being incompatible with intelligence to it being an evolutionary trade-off to suppress our rates of cancer. But Alvarado is optimistic it can be reactivated. Imagine, for example, being able to regrow a foot after it’s been amputated. Or repair a heart damaged by cardiac arrest.
“The fact that our molecular toolkit is almost indistinguishable to other vertebrates that can regenerate means it should be possible to engineer a means to introduce the ability of certain tissues to restore themselves. That, I think, is perfectly feasible! We’ll probably see that in the next 30 to 40 years.” More than enough time to also build a phone box that travels through time.
Verdict: We can’t do it now, but medical science might just allow us to regenerate in the near future.
About our expert, Alejandro Sánchez Alvarado
Dr Sánchez Alvarado is the Executive Director and Chief Scientific Officer of the Stowers Institute for Medical Research, USA. He researches the process and genetic control of regeneration and tissue maintenance.
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