An iron compass in the liver: superparamagnetic macrophages give homing pigeons a magnetic sense - Nature
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In a recent study published in Science, Lisowski et al. identified superparamagnetic, iron-loaded macrophages in the pigeon liver and showed that depletion of clodronate-sensitive phagocytic cells impaired homing under overcast skies, when solar cues were unavailable.1 Together, these observations point to a light-independent magnetoreceptive system outside the head and broaden current models of magnetoreception beyond receptors based on cryptochrome, magnetite, and ion channels in the eye and beak.2Magnetic measurements pointed the way (Fig. 1). Vibrating-sample magnetometry revealed a magnetic blocking transition near 12 K in liver and spleen, the signature of superparamagnetic nanoparticles, absent from muscle and beak. The measured signal could not be explained by isolated paramagnetic ferric iron, and low-temperature hysteresis indicated long-range magnetic order: not merely iron in the liver, but a biological nanomagnet. Prussian blue staining localized ferritin-bound iron to MHC II+ cells in hepatic sinusoids, abundant in liver, sparse in spleen, and absent from brain, eye, muscle, and beak. Fig. 1A liver-based route for magnetic orientation in homing pigeons.
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