No evidence for intracellular magnetite in putative vertebrate magnetoreceptors identified by magnetic screening.

The cellular basis of the magnetic sense remains an unsolved scientific mystery. One theory that aims to explain how animals detect the magnetic field is the magnetite hypothesis. It argues that intracellular crystals of the iron oxide magnetite (Fe3O4) are coupled to mechanosensitive channels that elicit neuronal activity in specialized sensory cells. Attempts to find these primary sensors have largely relied on the Prussian Blue stain that labels cells rich in ferric iron. This method has proved problematic as it has led investigators to conflate iron-rich macrophages with magnetoreceptors. An alternative approach developed by Eder et al. [Eder SH, et al. (2012) Proc Natl Acad Sci USA 109(30):12022-12027] is to identify candidate magnetoreceptive cells based on their magnetic moment. Here, we explore the utility of this method by undertaking a screen for magnetic cells in the pigeon. We report the identification of a small number of cells (1 in 476,000) with large magnetic moments (8-106 fAm(2)) from various tissues. The development of single-cell correlative light and electron microscopy (CLEM) coupled with electron energy loss spectroscopy (EELS) and energy-filtered transmission electron microscopy (EFTEM) permitted subcellular analysis of magnetic cells. This revealed the presence of extracellular structures composed of iron, titanium, and chromium accounting for the magnetic properties of these cells. Application of single-cell CLEM to magnetic cells from the trout failed to identify any intracellular structures consistent with biogenically derived magnetite. Our work illustrates the need for new methods to test the magnetite hypothesis of magnetosensation.

Subcellular analysis of pigeon hair cells implicates vesicular trafficking in cuticulosome formation and maintenance.

Hair cells are specialized sensors located in the inner ear that enable the transduction of sound, motion, and gravity into neuronal impulses. In birds some hair cells contain an iron-rich organelle, the cuticulosome, that has been implicated in the magnetic sense. Here, we exploit histological, transcriptomic, and tomographic methods to investigate the development of cuticulosomes, as well as the molecular and subcellular architecture of cuticulosome positive hair cells. We show that this organelle forms rapidly after hatching in a process that involves vesicle fusion and nucleation of ferritin nanoparticles. We further report that transcripts involved in endocytosis, extracellular exosomes, and metal ion binding are differentially expressed in cuticulosome positive hair cells. These data suggest that the cuticulosome and the associated molecular machinery regulate the concentration of iron within the labyrinth of the inner ear, which might indirectly tune a magnetic sensor that relies on electromagnetic induction.

Direct detection of a single photon by humans.

Despite investigations for over 70 years, the absolute limits of human vision have remained unclear. Rod cells respond to individual photons, yet whether a single-photon incident on the eye can be perceived by a human subject has remained a fundamental open question. Here we report that humans can detect a single-photon incident on the cornea with a probability significantly above chance. This was achieved by implementing a combination of a psychophysics procedure with a quantum light source that can generate single-photon states of light. We further discover that the probability of reporting a single photon is modulated by the presence of an earlier photon, suggesting a priming process that temporarily enhances the effective gain of the visual system on the timescale of seconds.

The Expression of Tubb2b Undergoes a Developmental Transition in Murine Cortical Neurons.

The development of the mammalian brain requires the generation, migration, and differentiation of neurons, cellular processes that are dependent on a dynamic microtubule cytoskeleton. Mutations in tubulin genes, which encode for the structural subunits of microtubules, cause detrimental neurological disorders known as the tubulinopathies. The disease spectra associated with different tubulin genes are overlapping but distinct, an observation believed to reflect functional specification of this multigene family. Perturbation of the ß-tubulin TUBB2B is known to cause polymicrogyria, pachygyria, microcephaly, and axon guidance defects. Here we provide a detailed analysis of the expression pattern of its murine homolog Tubb2b. The generation and characterization of BAC-transgenic eGFP reporter mouse lines has revealed that it is highly expressed in progenitors and postmitotic neurons during cortical development. This contrasts with the 8-week-old cortex, in which Tubb2b expression is restricted to macroglia, and expression is almost completely absent in mature neurons. This developmental transition in neurons is mirrored in the adult hippocampus and the cerebellum but is not a universal feature of Tubb2b; its expression persists in a population of postmitotic neurons in the 8-week-old retina. We propose that the dynamic spatial and temporal expression of Tubb2b reflects specific functional requirements of the microtubule cytoskeleton.

High resolution anatomical mapping confirms the absence of a magnetic sense system in the rostral upper beak of pigeons.

The cells that are responsible for detecting magnetic fields in animals remain undiscovered. Previous studies have proposed that pigeons employ a magnetic sense system that consists of six bilateral patches of magnetite containing dendrites located in the rostral subepidermis of the upper beak. We have challenged this hypothesis arguing that clusters of iron-rich cells in this region are macrophages, not magnetosensitive neurons. Here we present additional data in support of this conclusion. We have undertaken high resolution anatomical mapping of iron-rich cells in the rostral upper beak of pigeons, excluding the possibility that a conserved six-loci magnetic sense system exists. In addition we have extended our immunohistochemical studies to a second cohort of pigeons, confirming that iron rich cells in the upper beak are positive for MHCII and CD44, which are expressed by macrophages. We argue that it is important to critically assess conclusions that have been made in the past, while keeping an open mind as the search for the magnetoreceptor continues.

An iron-rich organelle in the cuticular plate of avian hair cells.

Hair cells reside in specialized epithelia in the inner ear of vertebrates, mediating the detection of sound, motion, and gravity. The transduction of these stimuli into a neuronal impulse requires the deflection of stereocilia, which are stabilized by the actin-rich cuticular plate. Recent electrophysiological studies have implicated the vestibular system in pigeon magnetosensation. Here we report the discovery of a single iron-rich organelle that resides in the cuticular plate of cochlear and vestibular hair cells in the pigeon. Transmission electron microscopy, coupled with elemental analysis, has shown that this structure is composed of ferritin-like granules, is approximately 300-600 nm in diameter, is spherical, and in some instances is membrane-bound and/or organized in a paracrystalline array. This organelle is found in hair cells in a wide variety of avian species, but not in rodents or in humans. This structure may function as (1) a store of excess iron, (2) a stabilizer of stereocilia, or (3) a mediator of magnetic detection. Given the specific subcellular location, elemental composition, and evolutionary conservation, we propose that this structure is an integral component of the sensory apparatus in birds.