Radial glia development in the mouse olfactory bulb

Radial glia are critical for cell migration and lamination of the cortex. I n most developing cortical structures, radial glia, as their name suggests, extend processes from the ventricle to the pia in regular parallel arrange ments. However, immunohistochemical labeling from several laboratories sugg ests that radial glia have a more branched morphology in the olfactory bulb . To investigate the morphology of radial glia in the mouse olfactory bulb we (1) labeled radial glia and olfactory receptor neuron axons at 24-hour i ntervals by immunohistochemistry; and (2) developed a novel method of gener ating and applying "nanocrystals" of 1,1 ' -dioctadecyl-3,3,3 ' ,3 '- tetra methylindocarbocyanine perchlorate (DiI) to the ventricle surface such that the processes of single olfactory bulb radial glia are labeled in the embr yonic olfactory bulb. We examined the structure and interactions of radial glia with ingrowing olfactory receptor neuron (ORN) axons in late embryonic olfactory bulb development. These results showed that olfactory bulb radia l glia do not form straight parallel structures as do radial glia in the ne ocortex but rather have a convoluted trajectory from the ventricle to the b ulb surface. Moreover, olfactory bulb radial glia consistently extend tange ntial branches at the level of the internal plexiform. layer. Beginning at embryonic day 17.5, two types of radial glia can be distinguished: type I r adial glia have a process that extends from the ventricle into the glomerul ar layer. These apical processes form highly restricted tufts, or "glial gl omeruli" at the same time that ORN axons are forming "axonal glomeruli." In type II radial glia the apical process does not enter the glomerular layer but instead ramifies within the external plexiform layer. The tight spatio temporal relationship between the glomerulization of radial glia processes and ORN axons during development suggest that radial glia processes could p lay a role in the formation and/or stabilization of mammalian glomeruli. (C ) 2001 Wiley-Liss, Inc.