Salmon, turtles and many birds migrate up to thousands of miles at a time, presumably by sensing the Earth's magnetic field. Now, scientists have identified cells in the nose of trout that respond to magnetism, offering a biological explanation for how animals orient themselves and find their way, even when it's dark or foggy.
The discovery -- and particularly the new method that enabled it -- opens up avenues for all sorts of futuristic applications, including miniaturized GPS systems or gene therapies that would restore sight, hearing or smell to people who have lost those senses.
The ability to detect magnetic-sensitive cells in the lab could also help answer questions about whether people are at risk from magnetic fields produced by power lines and other equipment.
"The key point is really the method we established. Some people call it a game-changer," said Michael Winklhofer, a biogeophysicist at the University of Munich. "Previously, we didn't have a tool to collect these cells. Now, we can do some serious cell biology on them."
"There's no doubt that many animals have a magnetic sense, particularly migratory birds and fish," he added. "But the problem is, we still don't know how that works."
Winklhofer and colleagues chose to study the olfactory tissues of trout based on decade-old research, which showed that magnetic fields affected the electrical activity of nerves that carried information from the fishes' noses. Instead of grinding up the tissues for analysis, as older methods tended to do, the researchers gently isolated whole cells from the tissues and put them into petri dishes.
When the team applied rotating magnetic fields to those dishes, about one out of every 10,000 cells spun with the same frequency as the fields, the researchers report today in the Proceedings of the National Academy of Sciences. Illuminated by the light of the microscope, structures inside of these cells also shone brilliantly, making them easy to detect.
A closer look revealed crystals attached to inside the cell membranes that contained what appeared to be magnetite, an iron-rich magnetic material. Scientists don't yet know how these structures work, but Winklhofer suspects that they excite membranes inside neurons and trigger nerve impulses that send direction-related information to the brain.
Based on the abundance of magnetic cells in the samples, Winklhofer estimated that each fish had a total of between 10 and 100 of these cells in its nose. As expected, there were no magnetic cells in the animals' muscle tissue. But in work yet to be published, his group detected even more magnetic cells in the trout's lateral line, a sensory organ in fish that detects vibrations.
Because magnetic fields penetrate the entire body, magnetic-sensing cells could be sporadically spread throughout in other body parts, too, which would make sense. If the cells were too close together, they would begin to sense each other's magnetic fields instead of the larger fields around the planet. Like needles in a haystack, though, magnetic cells can be difficult to find, which is what makes the new method so valuable.
The new technique also makes it possible to look for magnetic cells in animals that don't necessarily use a sense of magnetism but may have retained the cells even as evolution made them obsolete. In a 2008 study, for example, German researchers analyzed Google Earth images and saw that cows and deer tended to stand facing magnetic north or south.
Some recent research suggests that even people might harbor magnetic cells that linger from our ancestral hunter-gatherer days. If so, magnetic fields from power lines could be causing stress inside of our cells, leading to unknown health effects.
Researchers also hope to identify the genes and proteins responsible for producing magnetic-sensing cells, which would go a long way toward explaining how migrating animals accomplish such amazing feats. These discoveries would also pave the way for applications, such as tiny GPS systems or even novel strategies for healing blindness and other sensory problems in people.
"This may sound far out here," said J. David Dickman, a neuroscientist at the Baylor College of Medicine in Houston. "But we're contemplating taking cells that are not normally magnetically sensitive and creating cells that are magnetically sensitive. You could put them in the brain or body and turn them on or off with magnetic fields of certain wavelengths or frequencies to give balance or hearing back to the ear or smells back to the nose."
"Nature has a lot more yet to teach us," he added. "This study shows us that."