Abstract
Generation of neuronal diversity is a biological strategy widely used in the brain to process complex information. The olfactory bulb is the first relay station of olfactory information in the vertebrate central nervous system. In the olfactory bulb, axons of the olfactory sensory neurons form synapses with dendrites of projection neurons that transmit the olfactory information to the olfactory cortex. Historically, the olfactory bulb projection neurons have been classified into two populations, mitral cells and tufted cells. The somata of these cells are distinctly segregated within the layers of the olfactory bulb; the mitral cells are located in the mitral cell layer while the tufted cells are found in the external plexiform layer. Although mitral and tufted cells share many morphological, biophysical, and molecular characteristics, they differ in soma size, projection patterns of their dendrites and axons, and odor responses. In addition, tufted cells are further subclassified based on the relative depth of their somata location in the external plexiform layer. Evidence suggests that different types of tufted cells have distinct cellular properties and play different roles in olfactory information processing. Therefore, mitral and different types of tufted cells are considered as starting points for parallel pathways of olfactory information processing in the brain. Moreover, recent studies suggest that mitral cells also consist of heterogeneous subpopulations with different cellular properties despite the fact that the mitral cell layer is a single-cell layer. In this review, we first compare the morphology of projection neurons in the olfactory bulb of different vertebrate species. Next, we explore the similarities and differences among subpopulations of projection neurons in the rodent olfactory bulb. We also discuss the timing of neurogenesis as a factor for the generation of projection neuron heterogeneity in the olfactory bulb. Knowledge about the subpopulations of olfactory bulb projection neurons will contribute to a better understanding of the complex olfactory information processing in higher brain regions.
Highlights
Our ability to perceive the world through our senses begins with different sensory organs and results in distinct brain regions processing this information
In the rodent olfactory bulb (OB), the projection neurons located in the mitral cell layer (MCL) are defined as mitral cells, and others found in the external plexiform layer (EPL) and glomerular layer (GL) are deemed tufted cells (Ramon y Cajal, 1911; Shepherd et al, 2004; Greer et al, 2008; Ennis et al, 2015)
It was previously undetermined if external tufted cells project their axons outside the OB (Schoenfeld et al, 1985; Tobin et al, 2010; Lukas et al, 2019), Hirata et al (2019) recently showed that at least a subpopulation of external tufted cells do target the anterolateral edge of the olfactory tubercle (OT) as well as the pars externa of the anterior olfactory nucleus (AON)
Summary
The olfactory bulb projection neurons have been classified into two populations, mitral cells and tufted cells. Mitral and tufted cells share many morphological, biophysical, and molecular characteristics, they differ in soma size, projection patterns of their dendrites and axons, and odor responses. Evidence suggests that different types of tufted cells have distinct cellular properties and play different roles in olfactory information processing. Mitral and different types of tufted cells are considered as starting points for parallel pathways of olfactory information processing in the brain. We first compare the morphology of projection neurons in the olfactory bulb of different vertebrate species. Knowledge about the subpopulations of olfactory bulb projection neurons will contribute to a better understanding of the complex olfactory information processing in higher brain regions
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