Nelson Hall wants you to know that the googly-eyed flatworm he just sliced into four pieces is going to be OK. In fact, it's going to be great.

Three of the flatworm's four pieces have started to wriggle away from each other; its head is moving in circles under Hall's microscope.

"The head will just go off and do its own thing," says Hall, a doctoral student of bioengineering at Stanford University.

But within three weeks, the other pieces, as well as the head, will each have grown into a complete flatworm — identical to the one Hall sliced up — dark brown and about a half-inch long.

Hall and researchers around the world are hard at work trying to understand how most of a group of flatworms called planarians can use powerful stem cells to regenerate their entire bodies, an ability humans can only dream of.

When we suffer a severe injury, the best we can hope for is that our wounds will heal. But our limbs don't grow right back if they are cut off, the way that planarians regenerate.

"Healing is more like closing the wound and cleaning debris," Hall says. "It's too short of a process to have tissue replacement. Regeneration is replacing the tissue that was lost."

Other animals like starfish, salamanders and crabs can regrow a tail or a leg. Some planarians, on the other hand, can regrow their entire bodies — even their heads, which only a few animals can do.

Key to planarians' regenerative ability are powerful cells called pluripotent stem cells, which make up one-fifth of their bodies and can grow into every new body part. Humans only have pluripotent stem cells during the embryonic stage, before birth. After that, we mostly lose our ability to sprout new organs.

Humans do have a few types of tissue that can regenerate, says Dr. Stephen Badylak, deputy director of the McGowan Institute for Regenerative Medicine at the University of Pittsburgh. For example, the liver and bone marrow can do so, as can the outer layers of the skin and the inner layers of the intestine.

"But the way we heal most tissues is by forming scar tissue," Badylak explains.

Scientists hope that studying planarians could one day lead to treatments for humans in which some of our stem cells could be coaxed to regrow healthy limbs or organs to replace those that have been severed or otherwise damaged.

Doctors are limited in what they can currently do to help people who lose a limb or part of one. Badylak, who doesn't study planarians, has developed a treatment at the University of Pittsburgh that helps patients regrow their fingertips after an accident.

He does that by applying to the wounded finger a powder made of animal collagen and substances that stimulate cells to grow. The powder helps the finger to form a scaffold that attracts stem cells from the parts of the nail bed that weren't cut off. The stem cells regrow a fingertip. It isn't identical to the one that was cut off — it might be differently shaped. But it is functional.

Dr. Badylak and his team also have been able to help patients regrow 30 to 40 percent of the muscle they lost after a catastrophic injury caused by roadside bombs or motorcycle accidents. He says much more could be done with increased knowledge about stem cells, and he's excited by what scientists are learning about planarians.

"There's a tremendous amount to be gained by comparing the genes of regenerative species and nonregenerative species and seeing where the similarities and the differences are," Badylak said.

At Stanford University, Hall is working to make a green fluorescent planarian — one that would be genetically engineered with a protein to glow green under a certain type of light. This would allow researchers to insert different genes into planarians and study what those genes do.

"How do we genetically modify these worms so that we can put in our own genes," Hall wonders, "or remove existing genes to better understand how their regenerative programs function?"

A chunk of planarian with no tail and no head can regrow both in just three weeks, and the process is astounding to watch.

In the first week, two tiny spots appear on the piece of planarian: new eyes grown from scratch. But the planarian, if you can call it that yet, still looks like a blob.

By Day 12, it has grown a new head and a new tail — both translucent. They'll turn brown in another week. By now, it can eat, using a white, muscly tube called the pharynx, which operates something like a vacuum cleaner, extending out of the planarian's body and sucking up bits of food.

In ponds and springs where they're found in the wild, the planarians Hall studies feed on tiny animals and decomposing plants. But in the lab, they're fussier. Hall feeds them a beef liver paste — basically pâté.

Some types of planarians can use regeneration to reproduce without having sex. These asexual planarians break their bodies in two and grow a new planarian from each half. Within the same species there are also planarians that reproduce sexually, by laying eggs after mating.

"It's the same species that does both, which is kind of a weird thing," says biologist Dania Nanes Sarfati, a doctoral student at Stanford who is studying the flatworms' sexual organs.

Researchers haven't found much evidence that planarians in the wild use regeneration as a defense. Instead, scientists say, these flatworms seem to dissuade predators by coating themselves with an uninviting slime.

"In nature, they're not being cut up into fragments," says Ricardo Zayas, a neurobiologist at San Diego State University who studies planarians.

Zayas is studying how, after their heads are cut off in the lab, the planarians are able to regrow dozens of types of neurons that help them sense and interact with their environment.

"How do they smell food? How do they feel touch?" Zayas wonders. "And the neurons that are responsible for doing that — how do they regenerate and how do [the planarians] make them function?"

Scientists are trying to figure out exactly how planarians do it, in hopes that maybe one day the flatworms could inspire new ways to help us heal.

This post was produced by our friends at Deep Look, a wildlife video series from KQED and PBS Digital Studios that explores weekly "the unseen at the very edge of our visible world." Gabriela Quirós is the series' coordinating producer. AAAS Mass Media Fellow Allie Weill contributed reporting to this story. You can see more planarians in action here.

Copyright 2018 KQED. To see more, visit KQED.

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