Microglia in different layers of the rat olfactory bulb expressed a variety of membrane antigens except for CD4 (OX-35). Bulb microglial cells bearing complement receptor type 3 (OX-42) were ubiquitous and their immunoreactivity varied considerably in different bulb layers. Although very few in number, labeled microglia in all layers also expressed major histocompatibility complex class I antigen (OX-18), leukocyte common antigen (OX-1) and unknown macrophage antigen (ED-2). The latter was localized in cells distributed almost exclusively in the perivascular spaces. The immunoreactivity of ED-1, an unknown cytoplasmic or lysosomal membrane antigen in macrophages, was localized in labeled microglia which were concentrated mainly in the granule cell layer and periglomerular zone of the bulb. A variable number of microglial cells were stained with OX-6 (major histocompatibility complex class II antigen) and they were located predominantly in the periglomerular zone and at the junction between the granule cell layer and the subependymal layer. Ultrastructural study using GSA I-B4 lectin labeling showed that microglia in different layers of the bulb exhibited similar labeling patterns in their subcellular structures. A remarkable feature was the occurrence of some microglia in the olfactory nerve layer, subependymal layer and granule cell layer adjacent to the subependymal layer in which the cells showed intense lectin labeling at their Golgi apparatus, a feature which was absent in microglia of other bulb layers. Present results showed the regional differences in microglial antigen expressions and lectin labeling within the olfactory bulb. It is therefore suggested that the cells subserve very different specific functions depending on their ambient microenvironments. The heterogeneity of microglial functions in the olfactory bulb may be related to the progressive regeneration and degeneration of its containing neurons.
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