title: Ultrastructural analysis of a putative magnetoreceptor in the beak of homing pigeons.
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 microm in diameter and contain superparamagnetic magnetite (SPM) crystals. The SPM nanocrystals are assembled in clusters (diameter, approximately 1-2 microm). 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.
authors.affiliation: Zoologisches Institut, J. W. Goethe-Universität Frankfurt am Main, Siesmayerstrasse 70, D-60054 Frankfurt am Main, Germany. fleissner@zoology.uni-frankfurt.de
authors.forename: Gerta
authors.initials: G
authors.lastname: Fleissner
authors.forename: Elke
authors.initials: E
authors.lastname: Holtkamp-Rötzler
authors.forename: Marianne
authors.initials: M
authors.lastname: Hanzlik
authors.forename: Michael
authors.initials: M
authors.lastname: Winklhofer
authors.forename: Günther
authors.initials: G
authors.lastname: Fleissner
authors.forename: Nikolai
authors.initials: N
authors.lastname: Petersen
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Wiltschko
mesh_terms: D000818:Animals; D001502:Beak / Q000294:innervation* / Q000378:metabolism / Q000648:ultrastructure; D010856:Columbidae / Q000033:anatomy & histology / Q000502:physiology*; D005290:Ferric Compounds / Q000378:metabolism; D052203:Ferrosoferric Oxide; D006702:Homing Behavior / Q000502:physiology*; D007150:Immunohistochemistry; D007501:Iron / Q000378:metabolism; D008280:Magnetics; D008465:Mechanoreceptors / Q000378:metabolism / Q000648:ultrastructure*; D008854:Microscopy, Electron; D010087:Oxides / Q000378:metabolism; D011540:Prussian Blue Reaction; D014276:Trigeminal Nerve / Q000378:metabolism / Q000648:ultrastructure
chemical_list: D005290:Ferric Compounds; D010087:Oxides; D007501:Iron; C035885:ferric phosphate; D052203:Ferrosoferric Oxide