Planarians have unusual talents, to say the least. If you slice one of the tiny flatworms in half, the halves will grow back, giving you two identical worms. Cut a flatworm’s head in two, and it will grow two heads. Cut an eye off a flatworm — it will grow back. Stick an eye on a flatworm that lacks eyes — it’ll take root. Pieces as small as one-279th of a flatworm will turn into new, whole flatworms, given the time.
This process of regeneration has fascinated scientists for more than 200 years, prompting myriad zany, if somewhat macabre, experiments to understand how it is possible for a complex organism to rebuild itself from scratch, over and over and over again. In a paper published Friday in Science, researchers revealed a tantalizing glimpse into how the worms’ nervous systems manage this feat.
Specialized cells, the scientists report, point the way for neurons stretching from newly grown eyes to the brain of the worm, helping them connect correctly. The research suggests that cellular guides hidden throughout the planarian body may make it possible for the worm’s newly grown neurons to retrace their steps. Gathering these and other insights from the study of flatworms may someday help scientists interested in helping humans regenerate injured neurons.
María Lucila Scimone, a researcher at M.I.T.’s Whitehead Institute for Biomedical Research, first noticed these cells while studying Schmidtea mediterranea, a planarian common to bodies of freshwater in Southern Europe and North Africa. During another experiment, she noted that they were expressing a gene involved in regeneration.
“In every animal she looked at, she’d see just a couple of these, right next to the eye,” said Peter Reddien, a professor of biology at M.I.T. and also an author of the paper.
The team looked more closely and realized that some of the regeneration-related cells were positioned at key branching points in the network of nerves between the worms’ eyes and their brains. When the researchers transplanted an eye from one animal to another, the neurons growing from the new eye always grew toward these cells. When the nerve cells reached their target, they kept growing along the route that would take them to the brain. Removing those cells meant the neurons got lost and did not reach the brain.
The cells seemed to be acting as guides of some kind. Guidepost cells that point the way for other cells play important roles in embryo development in many creatures, Dr. Reddien said. But by the time most animals grow into adults, these cells are usually long-gone.
In flatworms, however, cells that perform this guiding role apparently exist in adults. They probably arrange themselves along the route from eye to brain using signals from muscle cells that tell them precisely where they should be in the body, Dr. Reddien said.
Scientists and doctors have long lusted after the regenerative powers of flatworms — not precisely with the goal of growing new heads, but of healing spinal cord damage and other serious injuries. Getting the right cells to grow to replace those lost is only part of the process, though.
“One of the things we’ve come to appreciate in this work is that the rewiring challenge could be a big one,” Dr. Reddien said. Ensuring that transplanted neurons wire themselves up correctly may be another important step.
In flatworms, Dr. Reddien and his colleagues are planning to continue looking for cells that give regenerating neurons a guide to follow.
“Are there guidepost-like cells in other parts of the nervous system?” he asked. Perhaps the nervous system is littered with tiny signposts, showing the way to the brain.