The research, performed at the University of Utah School of Medicine, was carried out by Helen Hay Whitney postdoctoral fellow Peter W. Reddien (now an Associate Member at the Whitehead Institute for Biomedical Research), and led by Howard Hughes Medical Institute investigator Alejandro Sánchez Alvarado.

Salamanders, zebrafish, and other organisms are capable of regenerating entirely new body parts. Although the human body does not face such demands, it is constantly replacing lost cells. For example, blood replenishes itself, wounds heal, and the lining of the gut sloughs off and is restored. Nowhere, however, is the process of regeneration more dramatic than in the freshwater flatworm planaria. Cut one of these animals in half, and a week later, two fully functional worms will have developed from the pieces. Cut a piece that is 1/279th the size of the animal, and it too will regrow into a complete worm.

The process, scientists know, is dependent on stem cells in the adult planaria known as neoblasts. Like all stem cells, neoblasts have the capability to develop into a variety of different cell types, meaning they can transform themselves into whatever tissue is needed after injury, be it intestine, skin, or brain. Even in the absence of injury, these cells are critical to maintain a healthy worm, as they are also responsible for replacing tissue that has been lost naturally. Scientists are just beginning to explore the molecular mechanisms that control adult stem cells, so, said Sánchez Alvarado, it's too soon to know how similar these mechanism are in planaria neoblasts and other organisms' stem cells. "But at least at the gross morphological level and gross biological functions, they compare quite well," he said.

Destruction of a planarian's neoblasts, which occurs when scientists expose the animal to radiation in the laboratory, is devastating. "The animal will survive on the virtue of its differentiated cells," Sánchez Alvarado said, "but as the tissues begin to turn over and there are no stem cells to replace such tissues, the animal begins, basically, to fall apart." It degenerates in a very specific way, he explained, with the tip of the head beginning to regress, followed by a curling up of the sides of the body. Not surprisingly, worms without neoblasts also lose their ability to regenerate.

With their unparalleled capacity for regeneration and the many environmental cues that influence the division and differentiation of their neoblasts, Sánchez Alvarado considers planaria an excellent model to tease out the intricacies of adult stem cell biology. "I think they probably have a lot to teach us about how a population of stem cells is being regulated in vivo, rather than in a Petri dish," he said. So Sánchez Alvarado and his colleagues set out to understand exactly how neoblasts carry out the remarkable maintenance and recreation of the varied tissues that make up a flatworm.

Earlier this year, they got their first clues when they individually turned off 1,065 of the worm's genes, and found 240 that were involved in some aspect of regeneration. Importantly, Sánchez Alvarado noted, 85 percent of these genes are found in the genomes of other organisms, including humans. In the current study, the scientists zeroed in on one of these genes, called smedwi-2, that was active in dividing neoblasts.

Read more: Howard Hughes Medical Institute