Ever since animals, such as lizards and starfish, were observed regenerating missing body parts, people have wondered where the new tissues come from. In the case of the planarian flatworm, Whitehead Institute researchers have determined that the source of this animal's extraordinary regenerative powers is a single, pluripotent cell type.

Most advanced animals, including mammals, have a system of specialized stem cells. In humans, we have blood stem cells in our bone marrow that make blood and immune cells, skin stem cells that produce new layers of skin, and intestinal stem cells that continually renew our gut linings, to name just a few. In humans, only embryonic stem cells and germ cells are pluripotent -- with the ability to create all cell types in the body.

In the planarian flatworm Schmidtea mediterranea, certain dividing cells, called neoblasts, have long been identified as essential for the regeneration that repairs the worm's tissues. Until now, however, scientists could not determine whether neoblasts represent a mixture of specialized stem cells that each regenerates specific tissues or are themselves pluripotent and able to regenerate all tissues.

"And that question is at the heart of understanding regeneration in these animals," says Whitehead Member Peter Reddien, who is also an associate professor of biology at MIT and a Howard Hughes Medical Institute (HHMI) Early Career Scientist. "The reason it's never been possible to address this question is because we needed assays that allow us to ask what the regenerative potential of single cells is."

Using complementary methods, Dan Wagner, Irving Wang -- two graduate students in the Reddien lab and co-first authors -- and Reddien have demonstrated that adult planarians not only possess pluripotent stem cells -- known as clonogenic neoblasts (cNeoblasts) -- but that a single such cell is capable of regenerating an entire animal. Their results are published in the May 13 issue of Science.

In one method, Wagner gave planarians a dose of radiation that killed all of their dividing cells, except for rare, isolated cNeoblasts. By labeling cells for a gene expressed only in neoblasts, Wagner observed that these individual surviving cNeoblasts divided to form large colonies of cells. Wagner analyzed the colonies and found that they contained cells differentiating into neurons and intestinal cells, indicating broad developmental potential for the initiating cNeoblast. Furthermore, Wagner showed that small numbers of cNeoblasts were capable of restoring regenerative potential to entire animals.

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Using another method, Wang and Reddien transplanted single cNeoblasts from one strain of planarian into lethally irradiated host planarians from a different strain, which lacked their own neoblasts and the ability to regenerate. Because the donor cells were distinguishable from the host, the researchers demonstrated that the transplanted cNeoblast multiplied, differentiated, and ultimately replaced all the host's tissues. From a single transplanted cell, the host not only regained the ability to regenerate, but was also converted to the genetic identity of the donor strain.

Because this work showed that cNeoblasts can differentiate into diverse tissue types and even replace all of the tissues in a host planarian, the researchers were able to conclude that cNeoblasts are pluripotent stem cells.

Further study of cNeoblasts could help researchers understand how stem cells can act to promote regeneration.

"This is an animal that, through evolution, has already solved the regeneration problem," says Wagner. "We're studying planarians to see how their regeneration process works. And, one day, we'll examine what are the key differences between what's possible in this animal and what's possible in a mouse or a person."

In the near future, the research group is interested in exploring the new possibilities provided by their techniques.

"Single-cell transplants have opened up a lot more experiments that we can do," says Wang. "Now that it is possible to identify and isolate single cNeoblasts, we can explore what makes these cells pluripotent."

This research was supported by the National Institutes of Health (NIH) and the Keck Foundation.

Peter Reddien's primary affiliation is with Whitehead Institute for Biomedical Research, where his laboratory is located and all his research is conducted. He is also a Howard Hughes Medical Institute Early Career Scientist and an associate professor of biology at Massachusetts Institute of Technology.