In a lab in MIT, a flatworm is dying. It’s a planarian – a simple animal that is normally very difficult to kill. Planarians are masters of regeneration; whole animals can be reborn from small clumps of tissue. If you cut one in half, it will simply grow into two planarians. But this animal has been bombarded with high doses of radiation that have wiped out its ability to regenerate. Slowly, its cells are bursting apart. With no new ones to replace them, the planarian has a few weeks to live. But Daniel Wagner and Irving Wang are about to save it, in a fashion. They transplant one special cell from a donor planarian into the terminal individual’s tail. The cell starts to divide. It produces skin, guts, nerves, muscle, eyes and a mouth. As the planarian dies from the head backwards, the transplanted cells spread from the tail upwards. At its worst, the animal is a stunted mass with no discernible head. But two weeks after the transplant, it has completely regenerated. A new planarian has risen, phoenix-like, from the ashes. Its entire body is now genetically identical to the single transplanted cell.
This ability to regenerate a body from a single cell is incredible, even by a planarian’s standards. It’s all the more unusual because the transplanted cell came from an adult. Of course, all animals grow from a single cell. Animal embryos are full of stem cells that can produce all the different tissues of the body, from muscle to skin to nerves. But adult stem cells are more limited. They can only produce cells from specific tissues. A blood stem cell, for example, can produce different types of blood cells but not neurons or skin. Planarians break the rule. Some of their adult cells can regenerate an entire animal, producing all the various types of cells that make up a planarian. Until now, scientists knew that planarians could regrow body parts using special cells called neoblasts. But no one knew whether there were separate neoblasts dedicated to specific tissues (like adult stem cells) or whether each neoblast had the infinite potential of embryonic stem cells. Wagner and Wang discovered the answer by exposing planarians to high levels of radiation that destroyed the vast majority of their dividing cells. But some survivors remained. Within a week, these special cells – known as c-Neoblasts – had produced larger clusters of dividing cells, spread throughout the animals’ bodies. Each cluster was a collection of clones, produced by a single ancestor. But even so, they contained different types of tissue – nerve cells, muscle cells, intestinal cells, and more. These c-Neoblasts were adult cells, but they seemed to have the limitless potential of embryonic stem cells. Just three or four clusters of these special cells were enough to regenerate an entire planarian after an otherwise lethal radiation dose. As the ultimate test of their abilities, Wagner and Wang transplanted single c-Neoblasts from one adult to another irradiated one. The procedure worked. The donor cells gradually took over the dying body of their host, producing a new animal. When Wagner and Wang cut the new planarian in half, the two halves could also regenerate. From a single adult cell, the duo created an entire family of planarians. Reference: Wagner, Wang & Reddien. 2011. Clonogenic Neoblasts Are Pluripotent Adult Stem Cells That Underlie Planarian Regeneration. Science http://dx.doi.org/10.1126/science.1203983