The planarian has a power few creatures can match. Remove its head, its tail or nearly any of its body parts, and this aquatic flatworm will simply grow it back. Humans can’t do that, of course. And yet many of the genes that help the planarian regenerate are also found in us. To learn more about this tiny marvel, we “interviewed” a representative.
“You could argue that I’m the perfect model for studying how to repair wounds or regrow limbs in humans,” says the flatworm. “But my place in science wasn’t always so secure.”
The planarian’s scientific career took off in the late 1800s, when the American geneticist Thomas Hunt Morgan showed that a planarian body fragment as small as 1/279th of the animal’s original size could regrow to form an entirely new worm. The planarian draws this astounding ability from neoblasts, stem cells that make up 20 to 30 percent of its body and can be deployed at any moment to repair a wound or regrow a severed body part. Morgan and other scientists at the time could see these cells, but they didn’t have the tools to explore how the cells worked. Without the necessary techniques to move regeneration research forward, scientists stopped studying the planarian.
That all changed in the late 1990s, when Alejandro Sánchez Alvarado, a scientist now at the Stowers Institute for Medical Research, went digging through scientific journals in search of experiments that would point him toward the perfect model for studying regeneration. He discovered the planarian and booked a flight to the Mediterranean, the worm’s home.
“I was in Barcelona, thinking my career was completely over, when Sánchez Alvarado and his colleague Phil Newmark fished me out of a murky fountain and put me back on the scientific map,” recalls the planarian.
The planarian traveled with Sánchez Alvarado to the United States, where it became the focus of a major gene sequencing effort. Today, Sánchez Alvarado and others continue to detect new planarian genes. They also use a technique called RNA interference (RNAi) to shut down its genes one by one to figure out which ones are necessary for regeneration.
The planarian is beginning to reveal some of the secrets behind its regenerative powers.
Scientists have pinpointed the specific gene that determines whether a head or tail will grow from a planarian wound site. Humans have this gene, too, but researchers don’t yet know what, if any, role it plays in healing.
Scientists have also identified the gene responsible for regenerating a planarian’s missing feeding tube. It turns out that this same gene helps form tissues in the digestive systems of mammals, including humans.
Researchers also recently found that the planarian excretory system functions very similarly to the most basic unit of the human kidney, the nephron . So in addition to being a good model for investigating regeneration, the planarian might also be a good model for studying human kidney conditions.
Despite its many contributions to science, the planarian remains humble.
“I have some pretty powerful regenerative abilities,” says the planarian. “But the truth is I’m not so different from you.”