In 1992 researchers identified the presence of magnetite—a permanently magnetic form of iron oxide—in human brain tissue. Iron in the body was no surprise. It is commonly found in ferritin, an intracellular protein common to several organisms, and the magnetite was thought to have formed biogenically, with some possibly originating in ferritin. But the presence of magnetite in the brain could be more than incidental. Various studies have shown that brain cells respond to external magnetic fields. There’s also a disturbing link to neurodegenerative disease: Evidence exists of elevated levels of magnetite in brain tissue from Alzheimer’s disease patients.
Now geophysicist Stuart Gilder, neuroscientist Christoph Schmitz (both at the Ludwig-Maximilians University of Munich), and their colleagues have carried out the first systematic mapping of magnetite nanoparticles in the human brain. At a magnetically shielded facility 80 km northeast of Munich, they used a superconducting magnetometer to measure the magnetic moments of hundreds of samples from seven dissected brains.
The chemical fixative used to store the brains is known to reduce the total iron concentration in tissue. Still, Gilder and company were able to measure any residual magnetization larger than 3.75 × 10−11 Am2. They found magnetite concentrated in the same places in all seven brains—primarily in the cerebellum and brain stem, as shown in the figure. A striking asymmetry also exists in the distribution of particles between the right and left hemispheres. The map establishes baseline data to which Gilder’s and other groups may compare later results as a function of variables such as age, gender, and neurological health. The map may also encourage studies of what function, if any, magnetite serves for humans. Magnetotactic bacteria, homing pigeons, and honeybees are among the organisms understood to sense magnetic field lines (see the article by Sönke Johnsen and Ken Lohmann in Physics Today, March 2008, page 29) with the same crystalline magnetite we humans have in our heads. (S. A. Gilder et al., Sci. Rep., 2018, doi:10.1038/s41598-018-29766-z.