Diffusion tensor imaging (DTI) and immunohistochemistry were used to examine axon injury in the rat spinal cord after unilateral L 2 -L 4 dorsal root axotomy at multiple time points (from 16 h to 30 d after surgery). Three days after axotomy, DTI revealed a lesion in the ipsilateral dorsal column extending from the lumbar to the cervical cord. The lesion showed significantly reduced parallel diffusivity and increased perpendicular diffusivity at day 3 compared with the contralateral unlesioned dorsal column. These findings coincided with loss of phosphorylated neurofilaments, accumulation of nonphosphorylated neurofilaments, swollen axons and formation of myelin ovoids, and no clear loss of myelin (stained by Luxol fast blue and 2Ј-3Ј-cyclic nucleotide 3Ј-phosphodiesterase). At day 30, DTI of the lesion continued to show significantly decreased parallel diffusivity. There was a slow but significant increase in perpendicular diffusivity between day 3 and day 30, which correlated with gradual clearance of myelin without further significant changes in neurofilament levels. These results show that parallel diffusivity can detect axon degeneration within 3 d after injury. The clearance of myelin at later stages may contribute to the late increase in perpendicular diffusivity, whereas the cause of its early increase at day 3 may be related to changes associated with primary axon injury. These data suggest that there is an early imaging signature associated with axon transections that could be used in a variety of neurological disease processes.

Inflammation, demyelination, gliosis and axonal degeneration are pathological hallmarks of multiple sclerosis (MS) and experimental autoimmune encephalomyelitis. Axonal damage is thought to contribute to irreversible damage and functional impairment, but is difficult to quantify. Conventional MRI has been used to assess the inflammatory and demyelinating aspects of MS lesions, but more sensitive and specific methods are needed to identify axonal damage to monitor disease progression and to determine efficacy of putative neuroprotective agents. We used high resolution diffusion tensor imaging (DTI) and fibre tracking to examine the spinal cord in rats with focal dorsal column inflammatory or demyelinating lesions to determine whether DTI measures can be used to detect pathology at the site of the focal lesion and to measure axonal damage in tracts distal to the focal lesion. Distant from the focal lesion, total axon counts, degenerating axon counts and SMI-31 staining, but not Luxol fast blue staining, were significantly correlated with fractional anisotropy, axial diffusivity and radial diffusivity, all of which are derived from the DTI data. These data suggest that high resolution DTI may be a more sensitive method than conventional imaging for detecting axonal damage at sites distant from inflammation.

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Parallel and perpendicular diffusion properties of water in the rat spinal cord were investigated 3 and 30 days after dorsal root axotomy, a specific insult resulting in early axonal degeneration followed by later myelin damage in the dorsal column white matter. Results from q-space analysis (i.e., the diffusion probability density function) obtained with strong diffusion weighting were compared to conventional anisotropy and diffusivity measurements at low b-values, as well as to histology for axon and myelin damage. q-Space contrasts included the height (return to zero displacement probability), full width at half maximum, root mean square displacement, and kurtosis excess of the probability density function, which quantifies the deviation from gaussian diffusion. Following axotomy, a significant increase in perpendicular diffusion (with decreased kurtosis excess) and decrease in parallel diffusion (with increased kurtosis excess) were found in lesions relative to uninjured white matter. Notably, a significant change in abnormal parallel diffusion was detected from 3 to 30 days with full width at half maximum, but not with conventional diffusivity. Also, directional full width at half maximum and root mean square displacement measurements exhibited different sensitivities to white matter damage. When compared to histology, the increase in perpendicular diffusion was not specific to demyelination, whereas combined reduced parallel diffusion and increased perpendicular diffusion was associated with axon damage. Magn Reson Med 63:1323-1335,

The study of CNS glial cell function requires experimental methods to detect, purify, and manipulate each cell population with fidelity and specificity. With the identification and cloning of cell- and stage-specific markers, glial cell analysis techniques have grown beyond physical methods of tissue dissociation and cell culture, and become highly specific with immunoselection of cell cultures in vitro and genetic targeting in vivo. The unique plasticity of glial cells offers the potential for cell replacement therapies in neurological disease that utilize neural cells derived from transplanted neural stem and progenitor cells. In this mini-review, we outline general physical and genetic approaches for macroglial cell generation. We summarize cell culture methods to obtain astrocytes and oligodendrocytes and their precursors, from developing and adult tissue, as well as approaches to obtain human neural progenitor cells through the establishment of stem cells. We discuss popular targeting rodent strains designed for cell-specific detection, selection and manipulation of neuroglial cell progenitors and their committed progeny. Based on shared markers between astrocytes and stem cells, we discuss genetically modified mouse strains with overlapping expression, and highlight SOX-expressing strains available for targeting of stem and progenitor cell populations. We also include recently established mouse strains for detection, and tag-assisted RNA and miRNA analysis. This discussion aims to provide a brief overview of the rapidly expanding collection of experimental approaches and genetic resources for the isolation and targeting of macroglial cells, their sources, progeny and gene products to facilitate our understanding of their properties and potential application in pathology.