Nature News Published online: 2 March 2006; | doi:10.1038/news060227-9
Surprise organ discovered in mice
After a century of scrutinizing the laboratory mouse, one might imagine that scientists would know the creature's body like the back of their own hands. Think again, because German researchers say they have discovered a whole new organ.
Common knowledge holds that in mice, the thymus, a pinkish-grey lump of tissue that helps to produce the infection-fighting T cells of the immune system, is roughly the size of a pea and nestles in the chest above the heart.
Now Hans-Reimer Rodewald at the University of Ulm in Germany, and his colleagues say they have discovered a smaller, second thymus hidden in the necks of lab mice. "I couldn't believe it for the first couple of months," Rodewald says. "2006 is not the time that you expect to change anatomy."
The discovery raises questions about some immunology studies in mice. Researchers interested in the immune system often slice out the main thymus of mice in order to study how this system works without it, and how T cells are produced in the gut and skin.
The presence of a second thymus suggests that many of these mice still had a working thymus in their necks, which could have confounded the results. These studies may need to be re-examined, Rodewald suggests: "Some people are not going to like this."
The discovery isn't a total surprise: biologists already knew that other mammals, including some humans, harbour an extra thymus in their necks. Studies from as early as the 1940s suggest that as many as five out of six human fetuses have a second thymus in the neck, says Clare Blackburn who studies the mammalian thymus at the University of Edinburgh, UK. There were also suggestions of a second mouse thymus in a report from the 1960s.
But Rodewald says that his team's study is the first to show that a second working thymus comes as standard in mice. "It's definitely not been part of people's thinking," agrees Blackburn.
Rodewald and his team stumbled across the long-overlooked organ when studying animals with defects in the chest thymus; they happened across tissue that looked suspiciously like that from the thymus but it was in the neck, next to the windpipe.
The team found that almost all healthy mice of certain strains have this second neck thymus, which is the size of a large pinhead. "It's like a pocket version," Rodewald says. It was probably overlooked until now because it looks remarkably similar to a lymph gland, he adds.
To prove the organ's identity, the researchers showed that it both looks and behaves like a thymus. They found that it holds cells only found in the thymus, and that, when transplanted into animals lacking any thymus of their own, can help generate T cells as part of an immune response.
Scientists are intrigued by where the second mouse thymus might come from. One idea is that it may be tissue left over from the formation of the main thymus, which grows in the neck region of the mouse embryo before moving down towards the chest. This suggests that it's "just pockets of tissue left behind as the organ migrates", Blackburn says.
Scientists are interested in the organ partly because they are keen to understand how the immune system develops. They also hope to identify stem cells from which the thymus sprouts, and perhaps use them to regenerate the organ in bone-marrow transplant patients whose thymus is damaged by treatment.
Could there be yet another thymus in mice? Chickens can have more than ten scattered down the neck. But Rodewald says, as far as mice go, "I'm not going to go for the third thymus."
