Glomerular formation in the developing rat olfactory bulb.

Using the confocal microscope together with markers for the cellular components of glomeruli, we examined the spatiotemporal cellular interactions that occur between the axons of olfactory receptor cells, their dendritic targets, 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-associated protein for double- and triple-label immunocytochemistry. Axons of olfactory receptor cells grew into a dense dendritic zone of the olfactory bulb (comprising the dendrites of both mitral and tufted cells) between E17 and E18. At E19, these axons coalesced into protoglomeruli, which continued to develop until birth, when the basic anatomical structure of adult glomeruli emerged. Neither mitral/tufted cell dendrites nor olfactory bulb astrocytes became specifically associated with these protoglomeruli until E21. Ensheathing cells remained restricted to the outer nerve fiber layer and did not appear to contribute to glomerular formation. Finally, the synaptophysin staining has shown that synaptic constituents are expressed as early as E17, prior to the appearance of mature olfactory receptor cell axons. Based on these data, we have established a time line detailing the temporal and spatial interactions that occur between cell types during late embryonic rat olfactory bulb development. We conclude that the initial event in the formation of glomeruli is the penetration of the mitral/tufted cell dendritic zone by olfactory receptor cell axons. The coalescence of dendritic and glial processes into glomerular structures appears secondary to the arrival of the olfactory receptor cell axons.