Ultrastructural analysis of a putative magnetoreceptor in the beak of homing pigeons

Fleissner, Gerta, Holtkamp-Rotzler, Elke, Hanzlik, Marianne, Winklhofer, Michael, Fleissner, Gunther, Petersen, Nikolai and Wiltschko, Wolfgang (2003) Ultrastructural analysis of a putative magnetoreceptor in the beak of homing pigeons. Journal of Comparative Neurology, 458 (4), 350-360. (doi:10.1002/cne.10579).

Abstract

With the use of different light and electron microscopic methods, we investigated the
subcellular organization of afferent trigeminal terminals in the upper beak of the homing
pigeon, Columba livia, which are about 5 m in diameter and contain superparamagnetic
magnetite (SPM) crystals. The SPM nanocrystals are assembled in clusters (diameter, 1–2
m). 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 delicate fiber strands. In addition to the SPM
clusters, a second inorganic iron compound has been identified: noncrystalline platelets of
iron phosphate (about 500 nm wide and long and maximally 100 nm thick) that occur along
a fibrous core of the terminal. The anatomic features suggested that these nerve endings
could detect small intensity changes of the geomagnetic field. Such stimuli can induce
deformations of the SPM clusters, which could be transduced into primary receptor potentials
by mechanosensitive membrane receptor channels. The subepidermal fat cells surrounding
the nerve endings prevent the inside from external mechanical stimuli. These structural
findings corresponded to conclusions inferred from rock magnetic measurements, theoretical
calculations, model experiments, and behavioral data, which also matched previous electrophysiologic
recordings from migratory birds.

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