No abstract

The polarity orientation of cellular microtubules is widely regarded to be important in understanding the control of microtubule assembly and microtubule-based motility in vivo . We have used a modification of the method of Heidemann and McIntosh (Nature (Lond .) . 286:517-519) to determine the polarity orientation of axonal microtubules in postganglionic sympathetic fibers of the cat. In fibers from three cats we were able to visualize the polarity of 68% of the axonal microtubules ; of these, 96% showed the same polarity orientation . Our interpretation is that the rapidly growing end of all axonal microtubules is distal to the cell body . We support Kirschner's hypothesis on microtubule organizing centers (J . Cell Biol. 86 : 330-334), although this interpretation raises questions about the continuity of axonal microtubules. Our results are inconsistent with a number of models for axonal transport based on force production on the surface of microtubules in which the direction of force is determined by the polarity of microtubules .

We report a novel approach for producing carbon nanotube fibers (CNF) composed with the polysaccharide agarose. Current attempts to make CNF’s require the use of a polymer or precipitating agent in the coagulating bath that may have negative effects in biomedical applications. We show that by taking advantage of the gelation properties of agarose one can substitute the bath with distilled water or ethanol and hence reduce the complexity associated with alternating the bath components or the use of organic solvents. We also demonstrate that these CNF can be chemically functionalized to express biological moieties through available free hydroxyl groups in agarose. We corroborate that agarose CNF are not only conductive and nontoxic, but their functionalization can facilitate cell attachment and response both in vitro and in vivo. Our findings suggest that agarose/CNT hybrid materials are excellent candidates for applications involving neural tissue engineering and biointerfacing with the nervous system.

Mordenite (MOR-type zeolite) is a widely used catalyst, in particular for (hydro-) isomerization and alkylation reactions in the petrochemical industry. However, having a one-dimensional micropore system, this material is susceptible to diffusion limitations and deactivation. To circumvent this problem, typically additional (meso)porosity is created by applying dealumination and/or steaming processes. The detailed description of the dealumination process is of crucial importance to understand how mordenite can be modified into an efficient catalyst. In this work, we present for the first time a simulation model to describe the influence of the dealumination process on the structural properties of mordenite. Using kinetic Monte Carlo simulations, dealumination is described as a multiple-step process consisting of the removal of the framework Al as well as the self-healing of silanol nests by Si atoms. The simulation results are in very good agreement with experimental results from 29 Si NMR, XRD, and N 2 and Ar physisorption. In particular, the simulations confirm the enlargement of the micropores and the creation of mesopores during dealumination.