Iron-rich tissue may serve as biological compass
Cells plucked from a trout’s snout swivel like tiny compasses to line up with a nearby magnet. That sensitivity, credited to iron inside the cells, could explain how fish, birds and other animals sense Earth’s magnetic field — a long-standing mystery among biologists.
“For decades scientists have been searching for the cells responsible for magnetosensation,” says David Keays, a neuroscientist at the Research Institute of Molecular Pathology in Vienna who wasn’t involved in the new study. “They're the biological equivalent of the elusive Higgs boson.”
The first demonstration of an animal’s internal compass dates to nearly half a century ago, when experiments showed that caged robins turn when exposed to rotating magnetic fields. Other birds, as well as sea turtles and some fish and amphibians, share this remarkable ability.
But the cells behind the unusual sense — which humans either lack or aren’t aware of — have remained elusive. Magnetic fields easily penetrate flesh, so receptors that respond to them could, presumably, be hidden anywhere in the body.
Recent clues have pointed to nose tissue as the place to look. In fish, magnetic fields can stimulate brain cells that connect to the nasal cavity, as demonstrated by Michael Walker, a neuroscientist at the University of Auckland in New Zealand, and colleagues. His team also found crystals of magnetite, nature’s most magnetic mineral, in nasal tissue taken from yellowfin tuna.
In the new study, reported online July 9 in the Proceedings of the National Academy of Sciences, Michael Winklhofer of the University of Munich and colleagues broke apart olfactory tissue from rainbow trout. Free-floating cells bombarded with magnetic fields were monitored for a response.
“Looking for magnetosensory cells was really like looking for a needle in a haystack,” Winklhofer says. Unlike the tightly packed receptors for taste, smell and sight, magnetic cells must be spread out. That’s because the magnetite that enables each cell to sense magnetic fields also generates a weak field itself, which could interfere with other cells.
Few and far between, these cells represented about one to four of every 10,000 cells, spun in a tight embrace with the rotating magnetic fields. A closer look at this microscopic tango revealed a chain of magnetite glued inside each cell’s membrane. Like a magnetized compass needle, the iron-rich mineral guided the cell around.
In living tissue, cells aren’t free to spin in this fashion. But the magnetite’s push could open up pores in a cell’s membrane. Charged particles moving in and out could set off electrical impulses, stimulating the brain. To support this theory, the researchers are looking for movements of charged calcium in living cells.
Meanwhile, Keays has built his own magnetoscope to search for magnetic cells in pigeons. He hopes to resolve an ongoing debate (SN: 5/19/12, p. 8) about whether the bird uses its eyes or ears to navigate Earth’s magnetic field — or whether, as it seems in the trout, its nose knows the way.
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