Magnetic Characterization of Isolated Candidate Vertebrate Magnetoreceptor Cells

Big news this week in the field of magnetoreception. A paper titled “Magnetic Characterization of Isolated Candidate Vertebrate Magnetoreceptor Cells” by Stephan Eder and his colleagues was published online (“early edition”) in the Proceedings of the National Academy of Sciences. One commentator went so far as to suggest that these magnetoreceptors are “the biological equivalent of the elusive Higgs boson” (an exaggeration, but a catchy quote with a grain of truth). The abstract to the paper is given below.

Over the past 50 y, behavioral experiments have produced a large body of evidence for the existence of a magnetic sense in a wide range of animals. However, the underlying sensory physiology remains poorly understood due to the elusiveness of the magnetosensory structures. Here we present an effective method for isolating and characterizing potential magnetite-based magnetoreceptor cells. In essence, a rotating magnetic field is employed to visually identify, within a dissociated tissue preparation, cells that contain magnetic material by their rotational behavior. As a tissue of choice, we selected trout olfactory epithelium that has been previously suggested to host candidate magnetoreceptor cells. We were able to reproducibly detect magnetic cells and to determine their magnetic dipole moment. The obtained values (4 to 100 fA m²) greatly exceed previous estimates (0.5 fA m²). The magnetism of the cells is due to a μm-sized intracellular structure of iron-rich crystals, most likely single-domain magnetite. In confocal reflectance imaging, these produce bright reflective spots close to the cell membrane. The magnetic inclusions are found to be firmly coupled to the cell membrane, enabling a direct transduction of mechanical stress produced by magnetic torque acting on the cellular dipole in situ. Our results show that the magnetically identified cells clearly meet the physical requirements for a magnetoreceptor capable of rapidly detecting small changes in the external magnetic field. This would also explain interference of ac powerline magnetic fields with magnetoreception, as reported in cattle.

The PNAS published a highlight about the article.

Identification of cells that sense Earth’s magnetic field

Researchers have isolated magnetic cells thought to underlie certain animals’ ability to navigate by Earth’s magnetic field. Behavioral studies have long provided evidence for the existence of a magnetic sense, but the identity of the specialized cells that comprise this internal compass has remained elusive. Stephan Eder and colleagues isolated the putative magnetic field-sensing cells that line the trout’s nasal cavity, and which contain iron-rich deposits of the magnetic material called magnetite. The authors placed a suspension of trout nasal tissue under a light microscope, and identified magnetic cells by their rotational motion in the presence of a slowly rotating external magnetic field. After siphoning off the rotating cells to characterize them in greater detail, the authors discovered that each cell contained reflective, iron-rich magnetic particles that were anchored to the cell membrane. The authors also determined that the cells are about 100 times more sensitive to magnetic fields than previously estimated. The findings suggest that the cells are capable of detecting magnetic north as well as small changes in the external magnetic field, and could form the basis of an accurate magnetic sensory system, according to the authors.

Russ Hobbie and I discuss the role of magnetic materials in biology in our chapter about biomagnetism in the 4th edition of Intermediate Physics for Medicine and Biology. We included in our book a photograph of magnetosomes (intracellular magnetite particles) in magnetotactic bacteria. In the photo, the magnetosomes are each about 0.05 μm on a side, and about 20 particles form a line roughly 1 μm long. Eder et al., on the other hand, find magnetic inclusions that are more spherical, and roughly 1–2 μm across. A trout cell has a really big magnetic moment when it contains such a large inclusion, but less than one cell in a thousand responds to the magnetic field and therefore presumably contains one. For a magnetotactic bacterium to have the same magnetic moment, it would need to be packed solid with magnetite.

I find the PNAS paper to be fascinating, and the method to detect individual cells using a rotating magnetic field is clever. However, in my opinion the last sentence of the abstract is a bit speculative, given that typical residential 60 Hz magnetic fields are 10,000 times smaller than the 2 mT fields used by Eder et al., and the frequency is almost 200 times higher. Granted the large magnetic moment makes the idea of powerline field detection intriguing, but that hypothesis is far from proven and I remain skeptical.

One of the coauthors on the PNAS paper is Joseph Kirschvink, whose work we discuss extensively in Section 9.10, Possible Effects of Weak External Electric and Magnetic Fields. Kirschvink is the Nico and Marilyn Van Wingen Professor of Geobiology at Caltech. He has developed several fascinating and controversial hypotheses, such as the Snowball Earth concept and the idea that a meteor found in 1984 contains evidence of life on Mars (he collaborated with my PhD advisor, John Wikswo, to make magnetic field measurements on that meteor). Kirschvink received the William Gilbert Award from the American Geophysical Union in 2011 for his work on geomagnetism. In the citation for this award, Benjamin Weiss of MIT wrote that “Joe represents everything we are looking for in a William Gilbert awardee. He is an ‘ideas man,’ a gadfly, working at the edge of the crowd while the crowd chases after him!” Kirschvink has also won Caltech’s Feynman Prize for Excellence in Teaching.

The PNAS paper has triggered an avalanche of press reports, including those in Science News,  the International Science Times, Science Daily, Phys.org, Live Science, and Discover Magazine.