Multiple system atrophy (MSA) is a neurodegenerative disorder that predominantly affects motor-related neuroanatomic structures. The role of microglia in MSA is unknown. To address this issue, we conducted quantitative image studies on the brains from 13 cases of MSA, comprising 8 cerebellar and 5 parkinsonian variants. Microglial and glial cytoplasmic inclusion (GCI) burdens were determined with image analysis on brain sections immunostained with antibodies to HLA-DR and alpha-synuclein. Many activated microglia, as well as GCIs, were noted in motor-related structures, including the cerebellar input, extrapyramidal motor, and pyramidal motor structures, but not in the cerebellar output structures. This result indicates that microglial activation, as well as the distribution of GCIs, is system-specific in MSA. The correlation analysis between the microglial and GCI burdens yielded variable yet significant correlations in the cerebellar input, extrapyramidal motor, and pyramidal motor systems, but not in the cerebellar output system. This result suggests that microglial activation is at least partly determined by GCIs or oligodendroglial alpha-synuclein in specific neuroanatomic systems affected in MSA. Taken together, considering the known toxic effects of microglia in neurodegenerative diseases, microglia may play a part in the development of system-specific tissue injuries, contributing to the system-bound clinical and pathological phenotypes.

Glial cytoplasmic inclusions (GCIs) and microglia were quantified in 12 cases of multiple system atrophy (MSA) with special reference to their association with histologically defined lesion severity. The targets of the analysis were white matter (cerebellum, pontine base) and gray matter (putamen, substantia nigra). First, the lesion severity was defined: for white matter, the degree of demyelination and tissue rarefaction were semi‐quantified on Klüver‐Barrera (KB) sections as grade I (mildly injured), II (moderately injured), and III (severely injured); for gray matter, neurons and astrocytes were counted on KB and glial fibrillary acidic protein‐immunostained sections, respectively. Next, the GCI burden was quantified on sections immunostained for α‐synuclein, phosphorylated α‐synuclein, and ubiquitin and the microglial burden was quantified on sections immunostained for HLA‐DR. In white matter, the GCI and microglial burdens were the greatest when the tissue injury was mild and/or moderate (grade I and/or grade II), and they became less prominent when the tissue injury became more severe (grade III). In gray matter, in contrast, the GCI and microglial burdens failed to show significant correlations with the lesion severity. Our result suggests that the amount of GCIs as well as that of microglia is reduced when the tissue injury becomes severe in vulnerable white matter areas, but not in vulnerable gray matter areas, of MSA. It also suggests that there seems to be a difference between gray matter and white matter in the way GCIs and microglia participate in the degenerative process of MSA.

Olig2, a member of the group of basic helix-loop-helix transcription factors, is innately expressed in oligodendrocytes. In the neoplastic condition, Olig2 is widely expressed in astrocytomas and oligodendrogliomas, but its expression in ependymomas remains poorly documented. A total of 59 brain tumors including 16 ependymomas, 32 astrocytomas, and 11 oligodendrogliomas were immunohistochemically studied for the expression of Olig2 as well as other markers including epithelial membrane antigen (EMA) and CD99. In general, the Olig2-positive nuclei were only sparsely distributed in ependymomas; in contrast, they were very numerous in astrocytomas and oligodendrogliomas. Particularly in cases of glioblastoma or pilocytic astrocytoma that histologically mimicked ependymoma, the Olig2-positive nuclei were numerous as in conventional astrocytomas, which helped to differentiate them from ependymomas. The EMA-positive structures were helpful for the diagnosis of ependymoma, however, they were occasionally very modest and sparse on immunostained sections. A quantitative study showed that the Olig2-positive nuclei were much fewer in ependymomas than in astrocytomas and oligodendrogliomas. These results indicate that the Olig2-immunohistochemistry is useful and potentially more reliable than the EMA-immunohistochemistry for the diagnosis ofependymoma. CD99 is a cell surface antigen expressed in some tumors, most notably in Ewing's sarcomas. In our preliminary experiment, we noted the absence of CD99-immunoreactivity in a fraction of brain tumors with clear cell morphology, including oligodendroglioma, clear cell ependymoma, and pilocytic astrocytoma (the oligodendroglioma-like component). Thus, we investigated the expression of CD99 in an additional series of brain tumors with clear cell morphology, including oligoastrocytoma (7 cases), central neurocytoma (6), and dysembryoplastic neuroepithelial tumor (9). We found that the absence of CD99-immunoreactivity was dependent on clear cell morphology rather than on tumor entities. The CD99-immunohistochemistry is unique in that it is helpful for the diagnosis of clear cell brain tumors through the visualization of CD99-negative clear cells.

A neuroepithelial tumor showing combined histological features of dysembryoplastic neuroepithelial tumor (DNT) and pleomorphic xanthoastrocytoma (PXA) is described. The patient was a 60-year-old male with a long-standing temporal lobe tumor and seizures. After a long, dormant period, the tumor, which had been localized in the left uncus, re-grew rapidly and extended into the subarachnoidal space and brain stem. The post-operative specimens disclosed two distinct components: an intra-cortical, cystic lesion containing mucinous materials and an extra-cortical, nodular lesion involving the leptomeninges. The former contained oligodendroglia-like small, round cells placed along axonal processes, plus mature neurons situated against mucinous materials (DNT-like component, WHO Grade I). The latter contained spindle and/or pleomorphic cells expressing glial fibrillary acidic protein, having bizarre nuclei and atypical mitotic figures. A reticulin network was developed among the tumor cells (PXA-like component, WHO Grade III). This case illustrates an unusual composite brain tumor, combined DNT and PXA.