Cut the head off and it grows back!
Slice the brain in two and both halves regrow, sharing past memories!
Run this creature through a mincer and, given time, a single organism becomes a horde of several hundred!
No, this is not the Borg from Star Trek or a blurb for a slasher zombie movie or some mutant from the scifi movie X-Men, this is the remarkable world of planarian flatworms.
Flatworms are unassuming creatures, seemingly insignificant in grand scheme of life on Earth, and yet they could hold the key to the future of regenerative medicine.
Look at a giraffe, a snake, a blue whale and a planarian worm, and it’s tempting to see entirely different, apparently unrelated animals. The truth is, they’re all powered by exactly the same engine at a cellular level: DNA.
In our day, Charles Darwin’s insight into Natural Selection and the Tree of Life has been confirmed by genetics. Not only has all of life descended from the same common ancestor billions of years ago, all of life shares the same basic mechanism for life.
The difference between a giraffe and a blue whale is not found in the length of their necks, or in the difference between fins and hooves, it’s found in the subtle arrangement of four different types of nucleobases in a single molecule, DNA. OK, this might be a little over simplistic in that there’s 3.3 x 10^9 base pairs in human DNA alone, but rearrange the nucleobases of a giraffe and technically you could come up with a blue whale.
Don’t let the term nucleobase fool you with an illusion of complexity, these are basic molecules, the chemical equivalent of lego blocks. In some cases, they have as few as nine atoms. And we’re not talking about exotic atoms like Polonium or Promethium being found in a nucleobase, there’s just the regular, old, garden varieties of Hydrogen, Oxygen, Nitrogen and Carbon.
Lego is a great analogy for DNA.
Rearrange the Lego used to build a toy house and you can build something entirely different, a toy car or a toy plane, all using the same pieces of Lego. In principle, the same is true of DNA.
The only difference when it comes to DNA is that DNA is the instruction set for how cells build the proteins that slowly support the increasing complexity that forms giraffes, blue whales, and planarians through a process not dissimilar to a Rube Goldberg machine.
Cellular life is astonishingly complex and multifaceted. Extending our Rube Goldberg analogy, DNA is the line of dominoes that topples a plastic cup, that knocks over a deck of cards, that releases a marble, that rolls down a spiral ramp and… you get the idea, only the cellular pathways that give rise to complex life occur millions of times a day inside every organism, from giraffes to snakes, blue whales to planarians, and even humble old Homo sapiens.
Why are planarians on the verge of revolutionizing regenerative medicine?
Because planarians are unique in being at the crossroads of two entirely different strategies for life. Planarians are complex animals, and yet they have the regenerative flexibility we see in the microbial world.
In the microbial world, slicing and dicing has no effect on an organism. Cells can live and divide indefinitely. In the macro world of multicellular animals we have specialized organs — hearts, lungs, eyes, brains, spinal cords and so on. These are far less robust.
Somewhere in our evolutionary history, multicellular animals traded the flexibility of the microbial world for the complexity of large-scale life, only planarians didn’t get the memo. Planarians have the best of both worlds.
Imagine cutting a human in half at the waist. Well, perhaps you don’t want to contemplate that outside of a magic trick as the implications are dire. Saw someone in half and they’ll undergo major trauma and die. A planarian, on the other hand, will regrow into two or more separate flatworms!
It’s as though our failed magic trick resulted in the legs growing a new trunk, arms and head, while the trunk grows a new set of legs, leaving us with two lovely assistants rather than one—twins!
Planarians accomplish this better-than-a-magic-trick using roughly the same genetic coding we have. We share about half our genes with planarians. They have a different number of chromosomes, but look close enough and you’ll see they have the same basic body plan we have, and the same chemistry at a cellular level. Planarians accomplish their “magic” using the same four nucleobases we see shared in the DNA of all life.
Many planaria genes resemble those of humans, and also many genes specifically linked to planarian stem cell biology and regeneration are conserved in humans
In theory, there’s no reason we couldn’t do the same thing if we could tap into the same mechanism at a cellular level, and that’s what makes planarians so fascinating.
Remember Jeeves from the science fiction movie Men in Black?
Jeeves was a human-looking alien with remarkable regenerative abilities.
The MIBs blow his head clean off his shoulders several times over the course of two movies, only to see it grow back in seconds. Jeeves complains, but is otherwise entirely normal. Well, as normal as an alien living undercover on Earth can be.
Planarians aren’t quite as quick, but they can pull off this trick with aplomb, as this image shows. Here, the head is severed completely, and yet regrows. It begins rewiring the neural pathways, developing new eyes, etc. And just like Jeeves, the resulting reformed planarian is entirely normal.
Physically, we might look vastly different from flatworms, but genetically we’re remarkably close. If you look at the entire Tree of Life, you’ll see humans (red) and flatworms (blue) are in the same basic region, sharing a surprisingly close common ancestor (green) when compared to the vast majority of life on Earth.
The hope is that human adult stem cells can one day be coaxed into the same astonishing proficiency we see in planarian flatworms, and not just from the perspective of replacing lost limbs but in terms of treating organ damage, spinal injury and aging.