A Limb Regeneration Mystery Solved

Salamanders have an enviable ability to regrow appendages that are amputated or injured; they re-create all the bones, muscle, skin, blood vessels, and nerves of the new body part so adeptly that it's hard to tell that it was ever missing. Because of this ability, salamanders have been popular subjects for scientists studying regeneration--and trying to learn how human cells might be coaxed to perform the same feat.

In salamanders, new tissues come from a tumorlike mass of cells that forms at the site of the injury, called the blastema. Until now, most scientists thought that the blastema contained a population of stem cells that had become pluripotent--capable of giving rise to all the needed tissues. But a new paper in the journal Nature provides evidence that this is not the case. Instead, stem cells involved in regeneration only create cells of the tissue that they came from. The finding suggests that regeneration does not require cells to reprogram themselves as dramatically as scientists had assumed.

Elly Tanaka, lead scientist of the study at the Center for Regenerative Therapies, in Dresden, Germany, says that "a lot of people had the impression that these blastema cells were all the same." Tanaka's lab had even shown previously that a single muscle fiber could give rise to several types of cells in a regenerated limb. But previous studies, she says, relied on imperfect methods of tracking cells, such as using fluorescent dyes that may have leaked out to other cells.

In the latest study, Tanaka's team employed a novel method for tracking the fate of cells from different tissues in a type of salamander called the axolotl. The researchers first created transgenic axolotls that carried green fluorescent protein (GFP) in their entire bodies. When the animals were still embryos, the researchers removed a piece of tissue from the limb region of the transgenic animals and transplanted the tissue into the same location in nontransgenic axolotls. The transplants were incorporated into the growing body as normal cells, and when the limb of the transplant recipients were then severed, the researchers could track the fate of the fluorescent cells as the limb regrew.

The researchers used this method to track the fate of cells of the inner and outer skin, muscles, and cartilage, as well as Schwann cells, which insulate nerve fibers. They found that, contrary to previous evidence, muscle cells at the amputation site only become muscle cells in the new limb. Other cell types also stuck to their previous identities; the only exception, Tanaka says, is that cells of the inner layers of skin and cartilage seem to be able to transform into one another. But for the most part, she says, the blastema is not a homogeneous mass of cells but "a mix of stem or progenitor cells from different tissues that stay separate during the whole process."