Connectivity of Phases and Growth Mechanisms in Peritectic Alloys Solidified at Low Speed: an X-Ray Tomography Study of Cu-Sn

The variety of microstructures that form at low solidification speed in peritectic alloys, bands, and islands, or even coupled (or cooperative) growth of the primary α and peritectic β phases, have been previously explained by nucleation-growth mechanisms. In a recent investigation on Cu-Sn, a new growth mechanism was conjectured on the basis of two-dimensional (2-D) optical microscopy and electron backscattered diffraction (EBSD) observations made in longitudinal sections. In the present contribution, synchrotron-based tomographic microscopy has been used to confirm this mechanism: α and β phases totally interconnected in three dimensions and bands (or islands) can result from an overlay mechanism, rather than from a nucleation events sequence. When the lateral growth of a new layer is too fast, an instability can lead to the formation of a lamellar structure as for eutectic alloy

Conserved microRNA editing in mammalian evolution, development and disease.

BACKGROUND: Mammalian microRNAs (miRNAs) are sometimes subject to adenosine-to-inosine RNA editing, which can lead to dramatic changes in miRNA target specificity or expression levels. However, although a few miRNAs are known to be edited at identical positions in human and mouse, the evolution of miRNA editing has not been investigated in detail. In this study, we identify conserved miRNA editing events in a range of mammalian and non-mammalian species. RESULTS: We demonstrate deep conservation of several site-specific miRNA editing events, including two that date back to the common ancestor of mammals and bony fishes some 450 million years ago. We also find evidence of a recent expansion of an edited miRNA family in placental mammals and show that editing of these miRNAs is associated with changes in target mRNA expression during primate development and aging. While global patterns of miRNA editing tend to be conserved across species, we observe substantial variation in editing frequencies depending on tissue, age and disease state: editing is more frequent in neural tissues compared to heart, kidney and testis; in older compared to younger individuals; and in samples from healthy tissues compared to tumors, which together suggests that miRNA editing might be associated with a reduced rate of cell proliferation. CONCLUSIONS: Our results show that site-specific miRNA editing is an evolutionarily conserved mechanism, which increases the functional diversity of mammalian miRNA transcriptomes. Furthermore, we find that although miRNA editing is rare compared to editing of long RNAs, miRNAs are greatly overrepresented among conserved editing targets

Nkx2.1 regulates the generation of telencephalic astrocytes during embryonic development.

The homeodomain transcription factor Nkx2.1 (NK2 homeobox 1) controls cell differentiation of telencephalic GABAergic interneurons and oligodendrocytes. Here we show that Nkx2.1 also regulates astrogliogenesis of the telencephalon from embryonic day (E) 14.5 to E16.5. Moreover we identify the different mechanisms by which Nkx2.1 controls the telencephalic astrogliogenesis. In Nkx2.1 knockout (Nkx2.1 <sup>-/-</sup> ) mice a drastic loss of astrocytes is observed that is not related to cell death. Further, in vivo analysis using BrdU incorporation reveals that Nkx2.1 affects the proliferation of the ventral neural stem cells that generate early astrocytes. Also, in vitro neurosphere assays showed reduced generation of astroglia upon loss of Nkx2.1, which could be due to decreased precursor proliferation and possibly defects in glial specification/differentiation. Chromatin immunoprecipitation analysis and in vitro co-transfection studies with an Nkx2.1-expressing plasmid indicate that Nkx2.1 binds to the promoter of glial fibrillary acidic protein (GFAP), primarily expressed in astrocytes, to regulate its expression. Hence, Nkx2.1 controls astroglial production spatiotemporally in embryos by regulating proliferation of the contributing Nkx2.1-positive precursors

Study of twinned dendrite growth stability

Under certain thermal conditions (G approximate to 1 x 10(4) K/m, v(s) approximate to 1 x 10(-3) m/s), twinned dendrites appear in aluminum alloys and can overgrow regular columnar dendrites, provided that some convection is also present in the melt. In order to check the stability of such morphologies, directionally solidified twinned samples of Al-Zn were partially remelted in a Bridgman furnace and then resolidified under controlled conditions, with minimal convection. It was found that, although twin planes remain stable during partial remelting, non-twinned dendrites regrow during solidification. They have a crystallographic orientation given by those of the twinned and untwinned "seed" regions, and grow along preferred directions that tend to be those of normal specimens. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Gene expression across mammalian organ development

The evolution of gene expression in mammalian organ development remains largely uncharacterized. Here we report the transcriptomes of seven organs (cerebrum, cerebellum, heart, kidney, liver, ovary and testis) across developmental time points from early organogenesis to adulthood for human, macaque, mouse, rat, rabbit, opossum and chicken. Comparisons of gene expression patterns identified developmental stage correspondences across species, and differences in the timing of key events during the development of the gonads. We found that the breadth of gene expression and the extent of purifying selection gradually decrease during development, whereas the amount of positive selection and expression of new genes increase. We identified differences in the temporal trajectories of expression of individual genes across species, with brain tissues showing the smallest percentage of trajectory changes, and the liver and testis showing the largest. Our work provides a resource of developmental transcriptomes of seven organs across seven species, and comparative analyses that characterize the development and evolution of mammalian organs

Modeling of peritectic coupled growth in Cu-Sn alloys

In directional solidification experiments on hypoperitectic Cu-Sn alloys at low velocity and high thermal gradient, both lamellar and fibrous coupled peritectic growth patterns have been observed. Two phenomena that had not been observed in previous experiments on other alloy systems are investigated here with the help of different modeling approaches. The mean volume fraction of primary phase alpha,(g) over bar (alpha), as determined by X-ray microtomography, decreases with solidification distance over the entire length of the coupled zone, but is always much larger than that expected from the equilibrium phase diagram. Moreover, oscillations in (g) over bar (alpha), with a spatial periodicity approximately equal to the lamellar spacing are also observed. The first observation is explained semi-quantitatively by a simple ID diffusion model, which reveals that the onset of coupled growth occurs during the initial transient of the primary phase planar front growth. A two-dimensional phase-field model is used to monitor the subsequent microstructure evolution, and shows that the lamellar structure exhibits collective 1-lambda oscillations. In agreement with previous studies, it was found that these oscillations lead to stable coupled growth only for a limited range of the control parameters. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved