Migration of newly generated neurons upon ependymally derived radial guide cells in explant cultures of the adult songbird forebrain

The adult songbird forebrain undergoes neuronal production throughout adulthood, with the production of new neurons in discrete regions of the neostriatal ventricular zone. Upon mitogenesis, these new neurons migrate into the subjacent brain parenchyma along radially directed guide fibers. In long-term ventricular zone explant cultures, derived from the higher vocal center of the adult canary, newly migratory neurons were found to associate preferentially with a characteristic substrate cell type. These small, parvonuclear substrate cells formed tightly packed epithelioid sheets, in which ciliated ependymal cells were common, as recognized by both live observation and electron microscopy. A subpopulation of these cells was immunostained by monoclonal antibody 3A7, which preferentially stains the guide fiber network of the adult avian brain. These 3A7+ cells included ependymal cells and bipolar radial cells, as well as morphologically defined astrocytes. As they matured in vitro, the 3A7+ bipolar radial cells extended long, unbranching fibers, which ultimately traversed the culture substrate. Like ependymal cells, they supported neuronal migration. These cells were likely homologous to radial guide cells in vivo. Thus, neuronal migration in adult avian forebrain culture occurred upon guide cells of ependymal derivation.     

Contrast-enhanced magnetisation transfer MRI in metastatic lesions of the brain

Our purpose was to compare prospectively the sensitivity of contrast-enhanced magnetisation transfer (MT) MRI and gradient-echo (GE) T1-weighted images in metastatic disease of the brain. We studied 52 patients with brain metastases, using conventional T1-weighted GE and MT spin-echo (SE) images after the same standard dose of gadolinium. Axial 5-mm reconstructions of GE data were compared with 5-mm MT images in the same plane. Metastases were counted independently by two neuroradiologists. In 12 patients (23%) MT imaging showed more metastases than GE images (P = 0.03). We detected 68 more metastases with the former technique.     

Multiexponential T2 relaxation in Fricke agarose gels: implications for NMR dosimetry

NMR relaxation times T1 and T2 of agarose and Fricke agarose gels have been measured in the range 17-51 MHz. The analysis of the spin echo curves indicates a multiexponential behaviour, characterized by three components, at all the examined frequencies. The relative T2 values, ranging from a few to a hundred milliseconds, can be attributed to different species of water molecules present in the gel. Two of these components are characterized by relaxation rates R2a and R2b, more sensitive than R1 to gamma irradiation, the sensitivity S being S(R1) = 0.066 s-1 Gy-1, S(R2a) = 0.088 s-1 Gy-1, S(R2b) = 0.17 s-1 Gy-1. The three T2 values decrease as a function of frequency, but no gain in dose sensitivity is obtained by changing the working frequency in the examined range. The relaxivity of agarose gels containing ferrous or ferric ions has also been measured and found to be different from those of the corresponding solutions in the absence of agarose. Thus it was possible to estimate the irradiation yield from three independent parameters, R1, R2a and R2b. No effect of the dose rate or of the source energy has been observed for any of these parameters.     

Tomsk complex for neutron-transmutation doping of silicon

All-Union Scientific-Research Institute of Nuclear Physics at the S. M. Kirov Tomsk Polytechnical University. Translated from Atomnaya énergiya, Vol. 79, No. 1, pp. 38–40, July, 1995.     

Mathematical model of external heat exchange in the flame space of a bath furnace for sheet glass

A mathematical model of complex heat exchange in the flame space of the melting zone of a regenerative bath furnace for sheet glass is presented. The calculated temperature fields in the gas space and on the internal surface of the roof, fields of the density of heat absorption by the surface of the pool. and consumption components in the heat balance of the furnace indicate the possibility of using the model for detailed analysis and prediction of its thermal operation.Translated from Steklo i Keramika, No. 3–5, May, 1996.     

Oscillation of the scattering time in a 2D electron system with oval antidots

Electron scattering by a single barrier is predicted to reveal singularities as the magnetic field is changed, because the number of electron collisions with the barrier dramatically increases as chaotic orbits around the barrier are changed into periodic orbits. To test this experimentally we have measured the magnetoresistance of AlGaAs/GaAs heterostructures with a two-dimensional electron gas and a lateral lattice containing a macroscopic number of oval-shaped antidots fabricated using electron lithography. Reproducible fluctuations in the magnetoresistance are observed at low field, which are due to the oscillations of the number of electron collisions with the antidots. The number of collisions N before the electron escapes from the antidot has been calculated as a function of B in an electric field. The position of the maxima in N(B) obtained from calculations and experiment are in reasonable agreement.