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Planaria: A window on regeneration
Both the head and tail have been removed from this planaria, S. mediterranea . This species is often used for research. Watch how both ends of the animal grow back in just eight days.

How do they do it?
Exactly how planarians work their regenerative magic is the million-dollar question that scientists would love to answer. Researchers have an inkling of some of the genes involved, but the picture is far from clear. What they do know is that the progress of regeneration can be followed by looking at certain features as they develop. (You can also follow this in the time-lapse movie at right.)

When a planarian loses its head (or its tail, or other chunk of itself), a regeneration blastema will begin to form at the site of the wound. The blastema is an area of whitish cells that are in an embryonic-like state, filled with stem cells that are able to become any of a number of kinds of cells. Over time, these cells will divide, more and more of them will differentiate, and the form of the missing body part will take shape. If scientists could understand how such cells get activated to make more of themselves, or to become the appropriate new cell types, they might learn how to trigger such responses in humans, allowing us to regrow our own tissues under certain conditions. There are many theories as to how planarians regenerate, but for the moment, we are far from understanding it—and even farther from knowing how to harness the potential in ourselves.

Regeneration and inheritance
Thomas Hunt Morgan
In the late 1800s, Thomas Hunt Morgan, the researcher who brought us fruit fly genetics, began his study of inheritance by looking at regeneration in planaria. The subject drew his interest because it formed a problem in a theory of inheritance put forth by German biologist August Weissman. Weissman’s idea, known as the germ plasm theory , postulated that each individual cell inherits only what material it needs to become a certain kind of cell. In other words, skin cells would contain only the material for skin cells, and so on. If this were true, Morgan argued, then regeneration wouldn’t be possible, and the headless body of a planarian wouldn’t be able to regenerate a brain.

We know today, of course, that all cells in an organism inherit the same complement of genetic material, and that stem cells drive the regeneration process. Those facts were unknown in the 1890s, though, and regeneration continued to baffle Morgan until he turned his attention to the fruit flies that made him famous. But his work wasn’t for naught: He published an informative volume on regeneration as well as discovering that he could cut the flatworm into a maximum of 279 pieces—and each one would grow into a new planarian.
Genes in common

Humans may not look much like flatworms, but there’s a surprising overlap between our genome and that of a planarian due to our distant yet common past. At least one of the genes we share is expressed in both planarian and human stem cells and is likely to be involved in regeneration. The planarian form is called piwi (pronounced PEE-wee), and the human form is known as hiwi . It turns out that planarian piwi is important for making stem cells that will divide to produce new, properly functioning daughter stem cells. In humans, the hiwi gene is expressed in sperm and eggs, as well as in some stem cells like those that generate new blood cells. Could it be part of a potential mechanism to trigger our own stem cells into action? The answer to that isn’t yet clear—we still know very little about the fundamental biology of our own stem cells. Sharing many of our genes, and masters of an art that is one of the most impressive in the living world, planaria offer a great model to learn from.