We previously reported that retrovirally marked clones in the mature chick diencephalon were widely dispersed in the mediolateral, dorsoventral and rostrocaudal planes. The current study was undertaken to define the migration routes that led to the dispersion. Embryos were infected between stages 10 and 14 with a retroviral stock encoding alkaline phosphatase and a library of molecular tags. Embryos were harvested 2.5-5.5 days later and the brains were fixed and serially sectioned. Sibling relationships were determined following PCR amplification and sequencing of the molecular tag. On embryonic day 4, all clones were organized in radial columns spanning the neuroepithelium, which was composed primarily of a ventricular zone at this age. No tangential migration was seen in the ventricular zone. On embryonic day 5, most clones remained radial with many cells located in the ventricular zone; however, a few clones had cells migrating perpendicular to the radial column, in either a rostrocaudal or dorsoventral direction. The tangential migration began just beyond the basal limit of the ventricular zone. On embryonic days 6 and 7, many clones had cells migrating perpendicular to the radial column, which spanned from the ventricular to the pial surface. The migrating cells appeared to be aligned along axes that were perpendicular to the radial column. Using a combination of DiI tracing, immunohistochemistry and electron microscopy, we have determined that axonal tracts are present and are aligned with the migrating cells, suggesting that they support the non-radial cell migration. These data indicate that migration along pathways independent of radial glia occur outside of the ventricular zone in more than 50% of the clones in the chick diencephalon.
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Objective: Pressure monitoring during injection has been suggested to help detect intrafascicle needle placement. We performed injections, guided by ultrasound, into the cervical nerve roots, peripheral nerves of the extremities, and perineural soft tissues of an unpreserved cadaver. We hypothesized that needle tip placement into these three sites would yield significantly different injection pressures, and that histologic analysis would allow comparison of intrafascicle vs. extrafascicle pressures of injection.Methods: Injections of 5 ml ropivacaine 0.5% were conducted at cervical roots (n=4), peripheral nerves of the extremities (n=10) and perineural soft tissues (n=9), at constant rate while monitoring pressure. Dye was instilled at the termination of the injection for histologic determination of needle position. Peak pressures and time to peak pressures, were compared for these three sets of injections. After microscopic examination, all intrafascicular and extrafascicular pressures were grouped together and compared.Results: Mean peak injection pressures for the three groups were found to be significantly different, (p=0.0002). At histological examination, four of 10 peripheral nerve injections resulted in deposition of dye within fascicles, while six of 10 did not. Mean peak intrafascicle injection pressures were significantly higher than those for injections outside of fascicles (p<0.0001). Time to peak injection pressure was not different for these two groups.Discussion: Comparison of intrafascicle versus extrafascicle injections showed a clear delineation of peak pressures into two ranges. This adds to prior evidence, from both human cadavers and live animals, showing that intrafascicle injections generate high pressures, whether conducted in nerve roots or peripherally.
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