Homing in on magnetism-sensing cells

A way in which cells can sense magnetic fields has been revealed in trout and might also hold the key to the human sense of direction...

For over 50 years it's been known that many species, humans included, are sensitive to magnetic fields and may use them to navigate.

But the biochemical basis of this internal compass has persistently eluded discovery. Deposits of iron in the beaks of pigeons were believed to account for these birds' homing abilities until they were shown recently to be aggregations of immune cells in what is the equivalent of avian "snot".

Now scientists have come up with a clever technique to pluck out magnetically-sensitive cells and probe them more deeply.

Writing in PNAS this week, Ludwig-Maximillians-University Munich researcher Stephan Eder and his colleagues took tissue from the noses of 50 trout, which are well-known to be a magnetically-sensitive species.

For each animal they dissociated the tissue to form a soup of individual cells in a dish. This was placed in a changing magnetic field that rotated once every three seconds.

Out of ten thousand cells in each suspension, between one and four cells could be seen rotating in sync with the applied magnetic field.

These cells were pulled out to study them further under the microscope. Intriguingly, each contained a small, bright crystalline blob about one thousandth of a millimetre across that was firmly attached to the inside of the cell's membrane. Analysed chemically, this was found to be rich in iron.

The researchers speculate that, when exposed to a magnetic field, these structures apply a force to the cell membrane, which can then be amplified and communicated to other cells, including the nervous system.

Encouragingly, as the team highlight in their paper, "the magnetic dipole moment of the candidate receptor cells is much larger than previously estimated and therefore not only sufficient to detect the direction of magnetic north but also to form the basis of an accurate magnetic sensory system from which to extract positional information from small spatial variations of geomagnetic field intensity and directions."