Magnetic characterization of isolated candidate vertebrate magnetoreceptor cells

Abstract

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 fAm(2)) greatly exceed previous estimates (0.5 fAm(2)). 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.

Fig. 1.

Time lapses of cell suspension from dissociated trout olfactory epithelium, showing individual cells rotating with magnetic field. (See Movies S1 and S2 for the two full sequences from which time lapses were extracted). (A) Transmitted light (T), showing an opaque inclusion (red arrow) in the rotating object. (B) Simultaneously recorded dark-field reflection (R) and fluorescence (FM1-43, lipophillic dye), showing reflective objects (white) and cell membrane (green). The rotating cell contains a strongly reflective inclusion (red arrow), displayed as close-up (upper right corner, scale bar 10 μm).

Fig. 2.

Confocal images of candidate magnetoreceptor cell. (A) Transmitted light (T). (B) Same cell viewed in confocal reflectance mode (R). Dashed yellow line indicates cell outline. Reflective inclusions inside the cell, with a close-up view (upper right window, scale bar: 2 μm). (C) Confocal fluorescence image showing DAPI labeling of the same cell. N: nucleus. Dashed yellow line: cell outline. (D) Composite image showing the nucleus and the reflective inclusions (red arrow). Scale bar: 10 μm.

Fig. 3.

Scanning electron microscopy of a previously rotating cell. Scale bars: 10 μm. (A) Backscattered electron image of the magnetic cell (left/vertical structure), which has lost cytoplasm (expelled to the right) during osmotic disintegration. The magnetic inclusion was retained (red arrow) and strongly backscatters electrons due to its high material contrast. (B) Secondary electron image does not reveal a contrast feature at the surface above the inclusion, demonstrating its intracellular nature. (C) Energy dispersive X-ray spectrum of the inclusion, showing a strong iron peak.

Fig. 4.

Measured magnetic dipole moment μ as a function of the rotating magnetic field amplitude H for 13 cells. The individual μ(H) measurements for a given cell are connected according to the measurement sequence. See Table S1 for numerical values.