In vivo serial MRI of age-dependent neural progenitor cell migration in the rat brain.

The subventricular zone (SVZ) is a neurogenic niche in the mammalian brain, giving rise to migratory neural progenitor cells (NPC). In rodents, it is well-established that neurogenesis decreases with aging. MRI-based cell tracking has been used to measure various aspects of neurogenesis and NPC migration in rodents, yet it has not yet been validated in the context of age-related decrease in neurogenesis. This validation is critical to using these MRI techniques to study changes in neurogenesis that arise in diseases prevalent in aging populations and their combination with advanced cellular therapeutic approaches aiming to combat neurodegeneration. As such, in this work we used MRI-based cell tracking to measure endogenous neurogenesis and cell migration from the SVZ along the rostral migratory stream to the olfactory bulb, for 12 days duration, in rats aged 9 weeks to 2 years old. To enable the specific detection of NPCs by MRI, we injected micron sized particles of iron oxide (MPIOs) into the lateral ventricle to endogenously label cells within the SVZ, which then appeared as hypo-intensive spots within MR images. In vivo MRI data showed that the rate of NPC migration was significantly different between all ages examined, with decreases in the distance traveled and migration rate as age progressed. The total number of MPIO-labeled cells within the olfactory bulb on day 12, was significantly decreased when compared across ages in ex vivo high-resolution scans. We also demonstrate for the first-time, provocative preliminary data suggesting age-dependent MPIO uptake within the dentate gyrus (DG) as well. Histology to identify doublecortin-positive NPCs, verified the decrease in cell labeling as a function of aging, for both regions. The dramatic reduction of NPC labeling within the SVZ observed with MRI, validates the sensitivity of MRI-based cell tracking to neurogenic potential and demonstrates the importance of understanding the impact of age on the relationship of NPC and disease.

In Vivo Microcomputed Tomography of Nanocrystal-Doped Tissue Engineered Scaffolds.

Tissue engineered scaffolds (TES) hold promise for improving the outcome of cell-based therapeutic strategies for a variety of biomedical scenarios, including musculoskeletal injuries, soft tissue repair, and spinal cord injury. Key to TES research and development, and clinical use, is the ability to longitudinally monitor TES location, orientation, integrity, and microstructure following implantation. Here, we describe a strategy for using microcomputed tomography (microCT) to visualize TES following implantation into mice. TES were doped with highly radiopaque gadolinium oxide nanocrystals and were implanted into the hind limbs of mice. Mice underwent serial microCT over 23 weeks. TES were clearly visible over the entire time course. Alginate scaffolds underwent a 20% volume reduction over the first 6 weeks, stabilizing over the next 17 weeks. Agarose scaffold volumes were unchanged. TES attenuation was also unchanged over the entire time course, indicating a lack of nanocrystal dissolution or leakage. Histology at the implant site showed the presence of very mild inflammation, typical for a mild foreign body reaction. Blood work indicated marked elevation in liver enzymes, and hematology measured significant reduction in white blood cell counts. While extrapolation of the X-ray induced effects on hematopoiesis in these mice to humans is not straightforward, clearly this is an area for careful monitoring. Taken together, these data lend strong support that doping TES with radiopaque nanocrystals and performing microCT imaging, represents a possible strategy for enabling serial in vivo monitoring of TES.