The cell in absence of aggregation artifacts.

Eduard Kellenberger understood that the conventional resin-embedding, he helped to develop (Ryter, A., Kellenberger, E., 1958. L'inclusion au polyester pour l'ultramicrotomie. J. Ultrastruct. Res. , 2, 200-214), was prone to aggregation artifacts (Kellenberger, E., 1987. The response to biological macromolecules and supramolecular structures to the physics of specimen cryo-preparation. In: Steinbrecht, R.A., Zierold, K. (Eds.), Cryo-techniques in Biological Electron Microscopy, Springer, Berlin, pp. 35-63). He was instrumental in developing various methods to overcome this limitation, for instance, by using low temperature-embedding and partially hydrophilic resins (Carlemalm, E., Garavito, R.M., Villiger, W., 1982. Resin development for electron microscopy and an analysis of embedding at low temperature. J. Microstruct., 126, 123-143; Villiger,W., 1993. Low temperature-embedding with Lowicryl resins. In: Robards, A.W., Wilson, A.J. (Eds.), Procedures in electron microscopy, Wiley, Chichester, UK, pp. 16:7.3-16:7.6).In principle, cryo-electron microscopy of vitreous sections is free of any aggregation artifact since the material remains fully hydrated and is free of chemical fixation or staining. The method is technically difficult still, but recent progress has made it amenable to routine practical applications.We compare here electron microscopical aspects of Zea mays meristem cells prepared by: (1) conventional resin-embedding and sectioning; (2) low temperature-embedding and sectioning of freeze substituted samples; and (3) cryo-sections of vitrified samples. The appearance of the extra-cellular space, the cytoplasm and the nucleoplasm are very different in conditions (1) and (3). They appear as compact, irregular and well delineated structures in conventional resin sections, whereas they are more diffuse and homogeneous in the vitreous sections. In the resin sections, the material seems to form a complex matrix, whereas it looks more like a thick soup in the vitreous sample. Low temperature-embedding (condition 2) shows an intermediate appearance. We suggest that regardless of the difference due to staining and different sectioning conditions, the other image differences are the consequence of aggregation artifacts in the resin-embedded specimens.     

VH-nuclei cosmic ray tracks in silicate minerals of howardites: A regolith stage of parent body formation

Natural tracks in pyroxene and plagioclase crystals from Erevan and Kapoeta howardites are due to irradiation by both galactic and solar cosmic rays.     

Dynamic phase transitions in cell spreading

We monitored isotropic spreading of mouse embryonic fibroblasts on fibronectin-coated substrates. Cell adhesion area versus time was measured via total internal reflection fluorescence microscopy. Spreading proceeds in well-defined phases. We found a power-law area growth with distinct exponents in three sequential phases, which we denote as basal, continuous, and contractile spreading. High resolution differential interference contrast microscopy was used to characterize local membrane dynamics at the spreading front. Fourier power spectra of membrane velocity reveal the sudden development of periodic membrane retractions at the transition from continuous to contractile spreading. We propose that the classification of cell spreading into phases with distinct functional characteristics and protein activity serves as a paradigm for a general program of a phase classification of cellular phenotype.     

Tunneling limit in the theory of photoelectron rescattering by the parent ion

A generalized Keldysh model is used to obtain simple analytical expressions for the energy and angular distributions of photoelectrons rescattered by the parent ion. The dependence of the form, absolute magnitude, and interference structure of the distributions on the parameters of the field and atom is investigated. It is shown that even though the semiclassical three-step rescattering model determines correctly the position of the boundaries beyond which the distributions decay rapidly, the model itself is inapplicable near them.     

Pair transfer in the absence of phase coherence

In his thesis, Josephson asserts that pair transfer accompanied by spontaneous photon emission does not demand the existence of a definite phase relationship between the two superconductors. In this letter we demonstrate the validity of this statement.     

Formation of dimeric parent cations in aromatic hydrocarbons

Third-order rate coefficients have been determined at 295 K for the formation of dimeric parent cations in benzene, benzene-d₆, toluene, the xylenes, mesitylene, and fluoro- and chlorobenzene using the NBS high pressure photoionization mass spectrometer. The reactions are all quite fast, the rate coefficients falling in the range 0.5–6 · 10⁻²⁵ cm⁶ molecule⁻² s⁻¹. The differences in rates observed are rationalized in terms of structural (geometric) considerations, and upper limits are established for the binding energies in (C₆H₆)₂⁺ and (C₆H₅CH₃)₂⁺. The temperature dependence for production of (C₆D₆)₂⁺ has been determined from 227 to 316 K, and the rate coefficient seems to obey a simple 1/T relationship over this range. The detection of trimeric species in benzene is also reported for the first time. Additional experiments involving benzene-nitric oxide mixtures are also reported, which include detection of an apparent equilibrium of the type NO⁺ + C₆H₆ ⇆ C₆H₆⁺ + NO.     

Nuclear moments of inertia in the absence of pairing

The nuclear moments of inertia in the absence of pairing are not equal to the rigid-body values because of quantal corrections due to the finite size of the system.     

Learning by message passing in networks of discrete synapses

We show that a message-passing process allows us to store in binary "material" synapses a number of random patterns which almost saturate the information theoretic bounds. We apply the learning algorithm to networks characterized by a wide range of different connection topologies and of size comparable with that of biological systems (e.g., [EQUATION: SEE TEXT]). The algorithm can be turned into an online-fault tolerant-learning protocol of potential interest in modeling aspects of synaptic plasticity and in building neuromorphic devices.     

The absence of ghosts in the dual fermion model

The consistency of the dual fermion model is further explored by showing the completeness of the transverse fermion physical states, in various situations, when the fermion ground state mass is zero. To this end the fermion physical state projection operator is constructed. A detailed direct proof of the absence of ghosts is given. The absence of fermion ghosts in twisted configurations is shown to require delicate cancellations. Comparison with the Neveu-Schwarz model indicates possible difficulties with the twisting operator.     

On the absence of supersymmetry anomalies in gauge theories

It is shown that N = 2 non-abelian gauge theory can be renormalized consistent with gauge invariance, supersymmetry and unitarity. There is no perturbative supersymmetry anomaly: the proof is independent of any specific regulator.     

On the absence of photodarkening in pnictide amorphous semiconductors

Parallel photodarkening experiments have been performed at 77 K on two glassy chalcogenides (As₂Se₃ and As₂Se_1.5Te_1.5 and one amorphous pnictide (α-As). Thermally reversible photodarkening effects are observed in both chalcogenides, but no photodarkening is observed in α-As even though α-As and As₂Se_1.5Te_1.5 have very similar band gaps. The negative result in α-As confirms the importance of non-bonding valence band electrons in the photodarkening process.     

On the absence of oxygen spectral lines in sputtering

It is well established that atomic oxygen spectral lines are normally not observed in optical studies of sputtering processes. We suggest that this can be explained in terms of local variations in the electron density along the solid surface. The model outlined is in accordance with the finding that the yields of secondary ions, secondary electrons, and secondary photons are normally enhanced by presence of oxygen at the target surface.     

Electronic correlations in CoO2, the parent compound of triangular cobaltates

A 59Co NMR study of CoO2, the x=0 end member of AxCoO2 (A=Na,Li,...) cobaltates, reveals a metallic ground state, though with clear signs of strong electron correlations: low-energy spin fluctuations develop at wave vectors q not equal to 0 and a crossover to a Fermi-liquid regime occurs below a characteristic temperature T* approximately 7 K. Despite some uncertainty over the exact cobalt oxidation state in this material, the results show that electronic correlations are revealed as x is reduced below 0.3. The data are consistent with NaxCoO2 being close to the Mott transition in the x-->0 limit.     

Tests of the absence of exotic exchanges

Many relations which sensitively express the suppression of exotic exchanged quantum numbers are derived and compared with the available data.