Hidden map of a worm to grow new eyes

Planarians have unusual talents, to say the least. If you cut one of the little flatworms in half, the halves will grow back and give you two identical worms. Cut the head of a flatworm in two, and it will grow two heads. Cut an eye out of a flatworm – it will grow back. Notice a flatworm that has no eyes – it will take root. Pieces as small as a 279 of a flatworm will become whole new flatworms, given time.

This regeneration process has fascinated scientists for more than 200 years, prompting a myriad of wacky, if somewhat macabre, experiments to understand how a complex organism can rebuild itself from scratch, over and over and over again. In an article published Friday in Science, the researchers revealed a tantalizing glimpse of how the worms’ nervous systems handle this feat.

The specialized cells, the scientists report, point the way for neurons to spread from the newly grown eyes to the worm’s brain, helping them to connect properly. Research suggests that hidden cellular guides throughout the planar body may allow the worm’s newly grown neurons to retrace their steps. Collecting these and other insights from the study of flatworms may one day help scientists interested in helping humans regenerate injured neurons.

Maria Lucila Scimone, a researcher at the Whitehead Institute for Biomedical Research at MIT, noticed these cells for the first time while studying Schmidtea mediterranea, a common planar to freshwater bodies in southern Europe and North Africa. During another experiment, she noticed that they were expressing a gene involved in regeneration.

“In every animal I looked at, I saw only a couple of these, right next to the eye,” said Peter Reddien, a biology professor at MIT and also the author of the article.

The team took a closer look and realized that some of the regeneration-related cells were located at key branch points in the nerve network between the worms’ eyes and their brains. When researchers transplanted one eye from one animal to another, the neurons that grow from the new eye always grew into these cells. When the nerve cells reached their target, they continued to grow along the path that would take them to the brain. By removing those cells, the neurons were lost and did not reach the brain.

The cells seemed to be acting as guides of some sort. Post-guide cells that point the way for other cells play important roles in embryonic development in many creatures, Dr. Reddien said. But by the time most animals become adults, these cells are already gone.

However, in flatworms, the cells that play this guiding role apparently exist in adults. They probably organize themselves along the route from the eye to the brain using signals from muscle cells that tell them exactly where they should be in the body, Dr. Reddien said.

Scientists and doctors have long wished for the regenerative powers of flatworms, not precisely with the goal of developing new heads, but to heal spinal cord damage and other serious injuries. However, getting the right cells to grow to replace the lost ones is only part of the process.

“One of the things we have come to appreciate in this work is that the rewiring challenge could be great,” said Dr. Reddien. Ensuring that transplanted neurons connect correctly can be another important step.

In flatworms, Dr. Reddien and his colleagues plan to continue searching for cells that give regenerating neurons a guideline to follow.

“Are there guide-type cells elsewhere in the nervous system?” I ask. Perhaps the nervous system is riddled with small signals that show the way to the brain.