Morphological Analyses of the Glomerulus in the Main Olfactory Bulb.

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Glomeruli of the main olfactory bulb (MOB) are now considered to serve as functional units in the olfactory information processing. Our previous analyses of the rat MOB revealed the compartmental organization of glomeruli and two types of periglomerular (PG) cells, type 1 (PG1)and type 2 (PG2) PG cells. We found that olfactory nerve axon terminals made synaptic contacts with dendritic tufts of both projection neurons (mitral and tufted cells) and PG1, but not of PG2, cells in the rat MOB. In the present project we extended our investigation to the fine structural organization of the glomeruli of mice (including genetically manipulated mice) and other mammals. Using the confocal laser scanning microscope and electron microscope, we found that the colocalization relationship of chemical substances in mouse PG cells was different from that of the rat PG cells reported previously.Furthermore, we confirmed the compartmental organization of the glomeruli and the presence of two types of PG cells in the MOB of bats, tree shrews, hedgehogs, moles, musk shrews and tenrecs. These structural features are therefore considered to be common and important organizational principles of the MOBs. We also exhibited complex neuronal interactions via chemical synapses and gap junctions in the glomerulus.