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What the humble planarian teaches us about the building blocks of life

Up close image of a planarian

By David Levin

At first glance, planarians aren’t exactly awe-inspiring. These brownish flatworms, each less than half an inch long, have few defining features: At one end, a tail comes to a rounded point; at the other, a head is punctuated by a pair of large, cartoonish eyes.

As unremarkable as the worms may seem, however, they have an ability we could only dream of. Cut off a planarian’s head, and it’ll sprout a new one. Lop off its tail, and it’ll soon be replaced. Even rending its body into tiny pieces is no big deal. Each chunk, regrowing from its wounded edges, replaces nerves, muscle, and other tissue until it becomes a separate, fully formed, individual worm in just a couple weeks.

“One of the big questions we want to answer is how it does this on a genetic level,” says Bo Wang, assistant professor of bioengineering at Stanford. “What’s in the worm’s DNA that lets it choose which body parts to grow, how much they’re going to grow, where to stop, and when to stop?”

Wang studies planarians’ alien-like regeneration. Part of what drives this astonishing ability, he says, is the fact that the worms’ bodies are filled with pluripotent stem cells, a type of “universal” cell that can grow into any type of tissue. When a worm is sliced in half, biochemical signals radiate outward from the damaged site, kicking those stem cells into action. Gradually, the cells differentiate into muscle, nerves, and other structures, forming a new head or tail over several days.