A few months ago I wrote about an article that disputed the claim that pigeons have iron-rich cells in their beaks that sense the earth’s magnetic field. A new paper by Eder et al. in Proceedings of the National Academy of Sciences describes their discovery of magnetic cells in the trout nose.
The way they discovered these cells was pretty ingenious. The authors took tissue from the trout olfactory epithelium, which is where chemical odors are sensed, and also where magnetic sensing probably occurs. They dissociated the cells, which means they separated them from each other, so they were free to move about in the liquid culture. Then they applied an external magnetic field and rotated it around the dish while they looked at the cells in the microscope. Out of every 10,000 cells, they observed 1-5 cells that rotated in sync with the magnetic field. Wow! I can imagine the excitement in the lab when they first saw a spinning cell. It’s no wonder that other labs were not able to isolate the magnetic-sensitive cells, since they are so sparse.
They noticed that each of these rotating cells had a dark chunk inside them that could reflect the microscope light. Upon closer inspection, this “inclusion” was located right next to the membrane just inside the cell. They analyzed the elemental composition of the inclusions and a major component was iron, the only biological atom that is magnetic. The authors suspect that the iron is in the form of magnetite (Fe3O4), which has been found in some bacteria.
The magnetic inclusions must be attached to the membrane, because the cells move at the same rate as the external magnetic field. If the magnetite were not tethered to the membrane, then it would spin freely in the intracellular liquid without affecting the rest of the cell.
|The cell on the left has unattached magnetite (Fe), whereas on the right it is attached to the membrane.
|How do spinning cells tell the rest of the trout about the location of the magnetic field? We don’t know, but when the cells are in the olfactory epithelium in the trout, I’m sure they will not be able to rotate so freely. What happens most likely is that changes to the magnetic field will cause the magnetite to change positions slightly, which will tug on the membrane and cause mechanoreceptors to open. These are ion channels that open or close when there are mechanical deformations of the membrane (like stretching or pushing). Once ion channels are involved, they can “activate” the cell and send signals to cells in the nervous system, which will relay this information to the brain. Of course, there’s no evidence that these particular rotating cells will do that in vivo, but it certainly is a tantalizing start.