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Laminar organization of mitral and tufted cells in the main olfactory bulb of the adult hamster

Morphometric analyses of Golgi‐impregnated mitral and tufted cells in the main olfactory bulb of the hamster indicate that these second‐order neurons cannot be considered a homogeneous population. They are comprised of several categories based on differences in the sizes and spatial organizations of their dendritic fields and in their likely patterns of lateral and recurrent interactions with granule cells and/or periglomerular cells. External tufted cells have their somata in the periglomerular region or at the superficial border of the external plexiform layer (EPL) and can be subdivided into three morphological categories. Many external tufted cells lack secondary dendrites, i.e., all of their dendrites arborize within glomeruli. Most of these neurons are not in positions where they could receive synaptic contacts from granule cells and thus are likely to receive interneuronal input predominantly from periglomerular cells. Another category of external tufted cells is characterized by secondary dendrites which are highly branched and form dense but spatially restricted fields in the EPL. In contrast, external tufted cells in the third category have sparsely branched secondary dendrites which extend tangentially in the EPL and tend to be asymmetrically distributed with respect to the soma. Most of the deeperlying tufted cells, and the mitral cells, have sparsely branched secondary dendrites whose lengths and laminar distributions within the EPL are correlated with the depths of their parent somata. Internal tufted cells, which have their somata in the deep one‐third of the EPL, and mitral cells exhibit the largest secondary dendritic fields. These fields extend tangentially within the EPL in relatively symmetric radial patterns from their parent somata and envelop substantial proportions of the EPL. Middle tufted cells have their somata in the superficial two‐thirds of the EPL and exhibit secondary dendritic morphologies which are intermediate between those of internal and external tufted cells. These differences in the size and spatial organization of secondary dendritic fields are likely to be associated with differences in the amount and spatial distribution of lateral and recurrent inhibition mediated through synaptic contacts with granule cells. External tufted cells which lack secondary dendrites have large glomerular arbors that are comparable in size to those of mitral and internal tufted cells, whereas external tufted cells with secondary dendrites and middle tufted cells tend to have relatively small glomerular arbors. The morphological differences among mitral and tufted cells and the likely differences in their synaptic interactions within the olfactory bulb, together with recent evidence that these neurons differ in their pharmacology and in their patterns of interconnections with more central regions of the forebrain, suggest that the mitral and tufted cells and the central olfactory circuits may be organized as functionally defined parallel pathways.