Glomerular formation in the developing rat olfactory bulb

Using the confocal microscope together with markers for the cellular compon ents of glomeruli, we examined the spatiotemporal cellular interactions tha t occur between the axons of olfactory receptor cells, their dendritic targ ets, and glial cells during the critical period of glomerular formation. We have employed markers of immature and mature olfactory receptor cell axons , mitral/tufted cell dendrites, and glial cells as well as a synapse-associ ated protein for double- and triple-label immunocytochemistry. Axons of olf actory receptor cells grew into a dense dendritic zone of the olfactory bul b (comprising the dendrites of both mitral and tufted cells) between E17 an d E18. At E19, these axons coalesced into protoglomeruli, which continued t o develop until birth, when the basic anatomical structure of adult glomeru li Emerged. Neither mitral/tufted cell dendrites nor olfactory bulb astrocy tes became specifically associated with these protoglomeruli until E21. Ens heathing cells remained restricted to the outer nerve fiber layer and did n ot appear to contribute to glomerular formation. Finally, the synaptophysin staining has shown that synaptic constituents are expressed as early as E1 7, prior to the appearance of mature olfactory receptor cell axons. Based o n these data, we have established a time line detailing the temporal and sp atial interactions that occur between cell types during late embryonic rat olfactory bulb development. We conclude that the initial event in the forma tion of glomeruli is the penetration of the mitral/tufted cell dendritic zo ne by olfactory receptor cell axons. The coalescence of dendritic and glial processes into glomerular structures appears secondary to the arrival of t he olfactory receptor cell axons. (C) 1999 Wiley-Liss, Inc.