Grains and grain boundaries in single-layer graphene atomic patchwork quilts

The properties of polycrystalline materials are often dominated by the size of their grains and by the atomic structure of their grain boundaries. These effects should be especially pronounced in two-dimensional materials, where even a line defect can divide and disrupt a crystal. These issues take on practical significance in graphene, which is a hexagonal, two-dimensional crystal of carbon atoms. Single-atom-thick graphene sheets can now be produced by chemical vapour deposition on scales of up to metres, making their polycrystallinity almost unavoidable. Theoretically, graphene grain boundaries are predicted to have distinct electronic, magnetic, chemical and mechanical properties that strongly depend on their atomic arrangement. Yet because of the five-order-of-magnitude size difference between grains and the atoms at grain boundaries, few experiments have fully explored the graphene grain structure. Here we use a combination of old and new transmission electron microscopy techniques to bridge these length scales. Using atomic-resolution imaging, we determine the location and identity of every atom at a grain boundary and find that different grains stitch together predominantly through pentagon-heptagon pairs. Rather than individually imaging the several billion atoms in each grain, we use diffraction-filtered imaging to rapidly map the location, orientation and shape of several hundred grains and boundaries, where only a handful have been previously reported. The resulting images reveal an unexpectedly small and intricate patchwork of grains connected by tilt boundaries. By correlating grain imaging with scanning probe and transport measurements, we show that these grain boundaries severely weaken the mechanical strength of graphene membranes but do not as drastically alter their electrical properties. These techniques open a new window for studies on the structure, properties and control of grains and grain boundaries in graphene and other two-dimensional materials.     

N-ethylmaleimide antagonizes stress-induced gastric ulcers in rats

N-ethylmaleimide (NEM) 10 or 25 mg/kg b.wt, given s.c. 20 min beforehand, dose-dependently and significantly antagonizes the severity of gastric glandular ulcers produced by restraint at 4 degrees C (stress) for 2 h. These findings suggest that reduced activity of endogenous nonprotein sulfhydryl substances in gastric tissue does not worsen stress-induced ulceration in rat stomachs, unlike the deleterious effect its depletion is claimed to have on ethanol-evoked gastric mucosal damage. Thus, decreased SH activity appears not to play a role in the aetiology of mucosal ulcers due to stress.     

Apolipoprotein-mediated pathways of lipid antigen presentation

Peptide antigens are presented to T cells by major histocompatibility complex (MHC) molecules, with endogenous peptides presented by MHC class I and exogenous peptides presented by MHC class II. In contrast to the MHC system, CD1 molecules bind lipid antigens that are presented at the antigen-presenting cell (APC) surface to lipid antigen-reactive T cells. Because CD1 molecules survey endocytic compartments, it is self-evident that they encounter antigens from extracellular sources. However, the mechanisms of exogenous lipid antigen delivery to CD1-antigen-loading compartments are not known. Serum apolipoproteins are mediators of extracellular lipid transport for metabolic needs. Here we define the pathways mediating markedly efficient exogenous lipid antigen delivery by apolipoproteins to achieve T-cell activation. Apolipoprotein E binds lipid antigens and delivers them by receptor-mediated uptake into endosomal compartments containing CD1 in APCs. Apolipoprotein E mediates the presentation of serum-borne lipid antigens and can be secreted by APCs as a mechanism to survey the local environment to capture antigens or to transfer microbial lipids from infected cells to bystander APCs. Thus, the immune system has co-opted a component of lipid metabolism to develop immunological responses to lipid antigens.     

Ultrastructural changes in rat liver cells after intramuscular implantation of a Walker tumor

Résumé L'implantation i.m. de la tumeur de Walker chez les rats provoque une dilatation, une désorganisation et une dégranulation progressives du réticulum endoplasmique granuleux des hépatocytes. Ces changements, qui s'accompagnent en même temps d'une prolifération du réticulum endoplasmique lisse, représenteraient les manifestations ultrastructurales d'une insuffisance microsomale démontrée biochimiquement dans le foie des rats affectés par la tumeur de Walker.

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This work was supported in part by the Ministère des Affaires Sociales, Québec, the Medical Research Council of Canada (Block Term Grant No. MT-1829), the Cancer Research Society Inc., and was undertaken as a special project of the Council for Tobacco Research, USA, and the Canadian Tobacco Industry.     

Cycloheximide-induced ultrastructural changes in the corpus luteum of rats

Résumé Le cycloheximide administré à des rats par voie i.v. modifie l'ultrastructure des cellules du corps jaune: ségrégation du nucléole, formation dans le cytoplasme de gros corps laminés et denses et agrégation des membranes lisses. On n'a pas encore établi si les modifications induites par le cycloheximide relèvent de l'inhibition de la synthèse des protéines ou d'une anomalie fonctionelle.

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Fellow of the Medical Research Council of Canada.