Magnetite in the head of the homing pigeon

Magnetite can be found in many life forms, including superparamagnetic(SPM) particles in the upper-beak skin of homing pigeons. Using transmission electron microscopy it has been found that these superparamagnetic particles are concentrated in the subcutis and are identified as aggregates of magnetite nanocrystals with grain sizes between 1 and 5 nm. These particles form clusters of 1— 3 μm diameter, which are distributed in distinct coherent elongated structures, associated with nervous tissue and located between fat cells. Studies on the subcellular organization of afferent trigeminal terminals in the upper beak of the homing pigeon based on opticaland electron microscopy have been reported. They are about 5μm in diameter and contain SPM crystals. The SPM nanocrystals are assembled in clusters , and about 10 to 15 of these clusters occur inside one nerve terminal, arranged along the cell membrane. Each SPM cluster is embedded in a solid fibrous cup open towards the cell surface, to which the cluster adheres by fiber strands. In addition to the SPM, iron phosphate is also found along a fibrous core of the terminal.The anatomic features suggest that these nerve endings can detect small intensity changes of the geomagnetic field. Several models about SPM based magnetoreceptor have been suggested.In this paper, a brief overview is presented of the above issues.