Thymus twice as nice for mice.

Researchers have long assumed that a mouse has only a single thymus. This organ, located directly over the heart, generates immune system components called T cells that protect the body from many pathogens. However, new research has turned up a second thymus, located in the neck.

While generating mutant mice for an unrelated study, Hans-Reimer Rodewald of the University of Ulm in Germany and his colleagues noticed that some of the altered animals had no thymus. However, the mice could still produce T cells, a job that only the thymus performs.

Searching for the T cells' source, Rodewald's team examined the animals' entire bodies. Eventually, they focused on an organ in the neck that looked like a lymph node. Unlike other lymph nodes, this organ showed activity in several genes previously known to function only in the thymus.

When the researchers transplanted these mysterious organs into other mice that had no evidence of thymus function, the animals began producing working T cells. The team reports these findings in an upcoming Science.

Rodewald notes that the findings could lead scientists to rethink previous experiments in which researchers removed thymuses from lab mice to stop them from making T cells. "We're not saying that those experiments don't hold up, but that you have to take this extra little thymus into consideration," he says.--C.B.