The emergence of compartmental organization in olfactory bulb glomeruli during postnatal development

The olfactory bulb glomerulus is a discrete and heterogeneous neuropil where olfactory receptor cell axons synapse with dendrites of mitral, tufted, and periglomerular neurons. To understand better the maturation of glomeruli and the spatiotemporal interactions that occur during postnatal development, we employed confocal microscopy and markers for immature and mature olfactory receptor cell axons in parallel with a marker for synaptic structure in maturing glomeruli. Sprague-Dawley rats at postnatal days 1, 6, 12, and 18 were processed for single- and double-label immunocytochemistry for olfactory marker protein (OMP), growth-associated protein (GAP-43), and synaptophysin. Mature or adult-like subcompartmental organization within the glomerulus emerged by postnatal day 12. Earlier in development immature axons entered the core of the glomerulus and moved to the periphery as they matured. However, beginning around 12 days postnatal, immature axons distributed in the periphery and moved toward the core as they matured. This change in the trajectories of axons into glomeruli suggests that different rules may be followed in establishing versus maintaining glomeruli. Double labeling with OMP and synaptophysin demonstrated strong colocalization compared with GAP-43 and synaptophysin, which showed much less colocalization, consistent with the notion that OMP is associated with more mature axons.