Odorant receptors form an ordered odorant map on the main olfactory bulb. This spatial representation disappears in most subsequent targets by “diffuse divergence and random convergence” of olfactory bulb projections. We revisited these projections using a newly developed method that can genetically dissect distinct subsets of olfactory bulb projection neurons based on their neuronal birthdates. Our birthdate tag analysis exposed parallel segregated projections formed by early-born mitral and late-born external tufted cells in otherwise apparently random olfactory networks. The results suggest that these parallel pathways extract unique features of information from the common olfactory input and process these features in a way similar to “color”, “orientation” or “direction” in the visual system. Importantly, the birthdate tag method can pave the way for deciphering the functional meaning of these individual pathways in the future.

Figure: Dissection of olfactory bulb projection neurons and their axon trajectories using neuronal birthdate tagging
Depending on tamoxifen injection stages (TM10.5~17.5), different classes of neurons such as accessory olfactory bulb neurons (AOB), mitral cells (MC) or tufted cells (TC) are tagged (pie charts), and their axon trajectories are revealed (bottom diagrams).

• This method is one of the basises of“NeuroGT Database”, a brain atlas ofneurogenic tagging CreER drivers”（NeuroGT）.