Global geometry optimization of clusters using a growth strategy optimized by a genetic algorithm

A new strategy for global geometry optimization of clusters is presented. Important features are a restriction of search space to favorable nearest-neighbor distance ranges, a suitable cluster growth representation with diminished correlations, and easy transferability of the results to larger clusters. The strengths and possible limitations of the method are demonstrated for Si10 using an empirical potential.Comment: accepted by Chem.Phys.Letters; 10 pages text, plus 3 pages for Title, abstract, and figure caption; figures 1a and 1

Tracing the vertical composition of disc galaxies through colour gradients

(Abbreviated) Optical observations of a statistically complete sample of edge-on disc galaxies are used to study the intrinsic vertical colour gradients in the galactic discs, to constrain the effects of population gradients, residual dust extinction and gradients in the galaxies' metal abundance. It appears that the intrinsic vertical colour gradients are either non-existent, or small and relatively constant as a function of position along the galaxies' major axes. Our results are consistent with the absence of any vertical colour gradient in the discs of the early-type sample galaxies. In most galaxies small-scale variations in the magnitude and even the direction of the vertical gradient are observed: at larger galactocentric distances they generally display redder colours with increasing z height, whereas the opposite is often observed in and near the galactic centres. For a significant fraction of our sample galaxies another mechanism in addition to the effects of stellar population gradients is required to explain the magnitude of the observed gradients. The non-zero colour gradients in a significant fraction of our sample galaxies are likely (at least) partially due to residual dust extinction at these z heights, as is also evidenced from the sometimes significant differences between the vertical colour gradients measured on either side of the galactic planes. We suggest that initial vertical metallicity gradients, if any, have likely not been accentuated by accretion or merging events over the lifetimes of our sample galaxies. On the other hand, they may have weakened any existing vertical metallicity gradients, although they also may have left the existing correlations unchanged.Comment: 17 pages LaTeX, incl. 5 embedded postscript figures, resubmitted to MNRAS (referee's comments taken into account

Morphometrics in Developmental Neurobiology: Quantitative Analysis of Growth Cone Motility in Vivo

In order for the nervous system to function properly, neurons in the brain must establish specific connections during embryonic development. Formation of neuronal circuits involves axons extending from cell bodies and navigating through diverse tissues to reach their targets in the brain. Once axons reach their target tissues, they arborize and make synaptic connections. Axon pathfinding is driven by dynamic motility behaviors expressed by terminal growth cones at the tips of the axons. Here, we applied morphometrics to determine quantitative values for six morphological and motility parameters for growth cones of optic axons navigating through the optic tract of a living brain preparation from a Xenopus laevis tadpole. Our results demonstrate an increase in length, decrease in width, increase in perimeter, decrease in area, increase in number of filopodia, and a decrease in number of lamellipodia, of the growth cones in the optic tract. Therefore, optic axonal growth cones become less circular and more elongated and protrusive during their navigation through the optic tract. Quantitatively deconstructing parameters of growth cone motility is necessary to determine molecular, cellular, and biophysical mechanisms of axon pathfinding, and to formulate computational analyses of developing neuronal connectivity in the brain

IFN-γ signaling, with the synergistic contribution of TNF-α, mediates cell specific microglial and astroglial activation in experimental models of Parkinson's disease

To through light on the mechanisms underlying the stimulation and persistence of glial cell activation in Parkinsonism, we investigate the function of IFN-γ and TNF-α in experimental models of Parkinson's disease and analyze their relation with local glial cell activation. It was found that IFN-γ and TNF-α remained higher over the years in the serum and CNS of chronic Parkinsonian macaques than in untreated animals, accompanied by sustained glial activation (microglia and astroglia) in the substantia nigra pars compacta. Importantly, Parkinsonian monkeys showed persistent and increasing levels of IFN-γR signaling in both microglial and astroglial cells. In addition, experiments performed in IFN-γ and TNF-α KO mice treated with MPTP revealed that, even before dopaminergic cell death can be observed, the presence of IFN-γ and TNF-α is crucial for microglial and astroglial activation, and, together, they have an important synergistic role. Both cytokines were necessary for the full level of activation to be attained in both microglial and astroglial cells. These results demonstrate that IFN-γ signaling, together with the contribution of TNF-α, have a critical and cell-specific role in stimulating and maintaining glial cell activation in Parkinsonism