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.