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Magnetic Cells: Stuff of Legend?

ANAHEIM, CALIFORNIA--Scientists have come a step closer to unraveling what appears to be an amazing ability of birds, bees, and fish to use Earth's magnetic fields to navigate. At the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW) here on 25 January, researchers claim to have found cells in the nose of a trout that act like a miniature compass.

Over the past 3 decades, researchers have found magnetite--a mineral used in the first humanmade compasses--in many life-forms. But locating the cells that contain the 50-nanometer particles has been like looking for a needle in a haystack, in part because the tissue-dissolving methods used to identify magnetite turn biological samples to mush. Two years ago, however, a team led by Michael Walker from the University of Auckland in New Zealand reported that captive trout could be conditioned to nudge a bar and receive food when they detected a magnetic field. They traced this magnetic sense to nerves rooted in iron-crystal-rich tissue located behind the trout's nose.

In their most recent work, Walker and colleague Carol Diebel turned to a magnetic force microscope, running it less than a hair's width above thin slices of trout nose tissue. Positive charges in a magnetic field attract the magnetized probe, and negative charges repel it. The trout snout lit up the computer screen, showing that the particles were polarized, much like a bar magnet. "We were jumping around the room when we saw this blip," Diebel says. The group next used an atomic force microscope to map structures within the putative magnetoreceptor cells. They discerned a u-shaped chain of magnetite within a fluid-filled sac resembling a vacuole of a cell. How this may function is anybody's guess; one theory holds that changes in Earth's magnetic field twist the magnetite chains, perhaps forcing open ion channels that send signals to the brain.

Scientists who have been engaged in a decades-long hunt for these cells--and have endured derision for working in a field tarnished by dubious research--say they feel vindicated. "This is the last nail in the argument," says Joseph Kirschvink, a geobiologist at the California Institute of Technology in Pasadena who first proposed the magnetite-based magnetoreception theory 20 years ago. But John Phillips, a neuroethologist at Indiana University, Bloomington, is cautious. To tie the mechanism to behavior, he says, researchers still must try to disrupt the cells and show an effect on navigation.