Amphipathic compounds containing N-acetylneuraminic acid (sialic acid) [for example, D-N-acetylneuraminyl-(alpha 2-1)-2S,3R,4E-2-N-tetracosanoyl sphingenine, sialyl alkyl glycerol ethers, and sialyl cholesterols] induced neuritogenesis in a neuroblastoma cell line (Neuro2a). The sialic acid in the hydrophilic moiety of the compounds is specifically required for neuritogenesis. The requirement for molecular specificity of the hydrophobic moiety, however, is rather low. Regarding the hydrophobic moiety, no preference for cholesterol, alkyl glycerol ether, or ceramide residues was observed as to their neuritogenic activity. Sialyl compounds with alpha-ketosidic sialyl linkages were more active than the corresponding beta-anomers. These sialyl compounds induced the growth of only one neurite, but a long one, from the cell body. This type of neuritogenes is completely different from that induced by compounds capable of elevating the concentration of intracellular cyclic AMP, which induced the appearance of many neurites from a single cell body. Besides this morphological change, the active sialyl compounds also caused a change in the carbohydrate composition of the cell surface. Sialyl compound treatment drastically increased the concentration of peanut agglutinin binding sites.

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The oxygen permeability of polycrystalline a-alumina wafers, which served as models for alumina scales on alumina-forming alloys, under steep oxygen potential gradients (DP O2 ) was evaluated at 1873 K. Oxygen permeation occurred by the grainboundary (GB) diffusion of oxygen from the higher-oxygenpartial-pressure (P O2 ) surface to the lower-P O2 surface, along with the simultaneous GB diffusion of aluminum in the opposite direction. The fluxes of oxygen and aluminum at the outflow side of the wafer were significantly larger than at the inflow side. Furthermore, Lu and Hf segregation at the GBs selectively reduced the mobility of oxygen and aluminum, respectively. A wafer with a bilayer structure, in which a Lu-doped layer was exposed to a lower P O2 and an Hf-doped layer was exposed to a higher P O2 , decreased the oxygen permeability. When the sign of DP O2 was reversed, however, the oxygen permeability of the wafer was comparable to that of a nondoped wafer. Co-doping with both Lu and Hf markedly increased the oxygen permeation, presumably because the Lu-stabilized HfO 2 particles that were segregated at the GBs acted as extremely fast diffusion paths for oxygen through the large number of oxygen vacancies introduced by the solid solution of Lu in the particles.

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