Magnetic resonance (MR) tracking of magnetically labeled stem and progenitor cells is an emerging technology, leading to an urgent need for magnetic probes that can make cells highly magnetic during their normal expansion in culture. We have developed magnetodendrimers as a versatile class of magnetic tags that can efficiently label mammalian cells, including human neural stem cells (NSCs) and mesenchymal stem cells (MSCs), through a nonspecific membrane adsorption process with subsequent intracellular (non-nuclear) localization in endosomes. The superparamagnetic iron oxide nanocomposites have been optimized to exhibit superior magnetic properties and to induce sufficient MR cell contrast at incubated doses as low as 1 microg iron/ml culture medium. When containing between 9 and 14 pg iron/cell, labeled cells exhibit an ex vivo nuclear magnetic resonance (NMR) relaxation rate (1/T2) as high as 24-39 s-1/mM iron. Labeled cells are unaffected in their viability and proliferating capacity, and labeled human NSCs differentiate normally into neurons. Furthermore, we show here that NSC-derived (and LacZ-transfected), magnetically labeled oligodendroglial progenitors can be readily detected in vivo at least as long as six weeks after transplantation, with an excellent correlation between the obtained MR contrast and staining for beta-galactosidase expression. The availability of magnetodendrimers opens up the possibility of MR tracking of a wide variety of (stem) cell transplants.

We rcpmt results on the magnetic properties of magnetites produced by magnetotaetie and dissimilatory iron-reducing bacteria. Magnetotactic bacterial (MTB) strains MS I. MV J and MV2 and dissimilatory iron-reducing bacterium ,train GS-15. grown in pure cultures, were used in this study, Our results suggest that a comhination of room tempnature coercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain (SD) particles of magnetite from MTBs. The most useful magnetic property measurements include: (1) acquisition and demagnetization of isothermal remanent magnetiza tion (IRM) using static, pulse and altcrnating fields: (2) acquisition of anhysteretie remanent magnetization (ARM); and (3) thermal dependence of low temperature (20 K) saturation IRM after cooling in zero field (ZFC) or in a 2.5 T field (Fe) lrom 300 K, Howcver, potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference hetween low temperature zero-field and field cooled 51 RMs on warming through the Verwey transition (T z J00 K). Intact chains of unoxidized magnetite magnetoSl)ll1eS have ratios of D,.c/t5ZFC greater than 2, where the parameter D is a measure of the amount of remanenCl' lost by warming through the Verwey transition. Disruption of the chain structure or conversion of the magnetosuilles to maghemite reduces the.) FC/OLl-C ratio to around L similar to values observed for some inorganic magnetite. rnaghemitc. greigitc and GS-15 particles. Numerical simulations of t5,.c/Dzvc ratios for simple binary mixtures of magnctosome chains and inorganic magnetic fractions suggest that the 15 njD zFc parameter can he a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomcs and can he a useful magnetic technique for identifying them in whole-sediment samples. The strength of our approach lies in the comparati\e case and rapidity with which magnetic measurements can be madc, compared to techniques such as electron III ic roscopy.

At the Verwey transition (Tv≈110–120 K), magnetite transforms from monoclinic to cubic spinel structure. It has long been believed that magnetic remanence and susceptibility would change markedly at Tv in the case of coarse grains but only slightly or inappreciably in the case of fine (<1 µm) grains. We find on the contrary that remanence changes at Tv by 50–80% in both large and small crystals, if they are stoichiometric. However, minor surface oxidation suppresses the transition, and the fact that fine grains oxidize more readily leads to an apparent size dependence. Our experiments used submicron magnetite cubes with mean sizes of 0.037, 0.076, 0.10 and 0.22 µm which were initially non‐stoichiometric (oxidation parameter z from 0.2–0.7). A saturation isothermal remanent magnetization (SIRM) given in a 2.5 T field at 5 K decreased steadily during zero‐field warming to 300 K with little or no indication of the Verwey transition. After the oxidized surface of each crystal was reduced to stoichiometric magnetite, the SIRM decreased sharply during warming by 50–80% around 110 K. The change in SIRM for the 0.22 µm grains was almost identical to that measured for a 1.5 mm natural magnetite crystal. Thus a 1012 change in particle volume does not materially affect the remanence transition at Tv but oxidation to z=0.3 essentially suppresses the transition. The effect of the degree of oxidation on Tv provides a sensitive test for maghemitization in soils, sediments and rocks.

No abstract

No abstract