Topographically distinct projection patterns of early- and late-generated projection neurons in the mouse olfactory bulb

Abstract

In the mouse brain, olfactory information is transmitted to the olfactory cortex via olfactory bulb (OB) projection neurons known as mitral and tufted cells. Although mitral and tufted cells share many cellular characteristics, these cell types are distinct in their somata location and in their axonal and dendritic projection patterns. Moreover, mitral cells consist of heterogeneous subpopulations. We have previously shown that mitral cells generated at different embryonic days differentially localize within the mitral cell layer and extend their lateral dendrites to different sublayers of the external plexiform layer. Here, we examined the axonal projection patterns from the subpopulations of OB projection neurons that are determined by the timing of neurogenesis (neuronal birthdate) to understand the developmental origin of the diversity in olfactory pathways. We separately labeled early- and late-generated OB projection neurons using in utero electroporation performed at embryonic day 11 and 12, respectively, and quantitatively analyzed their axonal projection patterns in the whole mouse brain using high-resolution 3D imaging. In this study, we demonstrate that the axonal projection of late-generated OB projection neurons is restricted to the anterior portion of the olfactory cortex while those of the early-generated OB projection neurons innervate the entire olfactory cortex. Our results suggest that the late-generated mitral cells do not extend their axons to the posterior regions of the olfactory cortex. Therefore, the mitral cells having different birthdates differ, not only in cell body location and dendritic projections within the OB, but also in their axonal projection pattern to the olfactory cortex.

Significance Statement The olfactory bulb contains long-range projection neurons with distinct connectivity to higher order brain regions. Here, we examined how the birthdate of the olfactory bulb projection neurons correlates to the generation of differential connectivity patterns. We used in utero electroporation and high-resolution 3D imaging of the whole mouse brain, and determined the topographically distinct axonal projection patterns of early- and late-generated olfactory bulb projection neurons. Our results show that the timing of neurogenesis is a determining factor for the innervation of olfactory bulb projection neurons and indicate that mitral cells having different birthdates are the origins of distinct olfactory information pathways. Our study provides novel insights into the formation of neuronal circuits processing multiple aspects of olfactory information.