Neurogenesis and synaptogenesis
The olfactory system has the unusual capacity to generate new neurons throughout the lifetime of an organism. This allows to study stem cell function and the regulation of neurogenesis in the developing and adult olfactory system. The olfactory system generates new neurons throughout lifetime. Neurogenesis occurs both in the olfactory organ (see Hassenklöver et al., 2009) and the olfactory bulb (Hassenklöver et al., 2010), the brain region responsible for primary odor processing. Olfactory stem cell in the basal portion of the olfactory epithelium give rise to new olfactory receptor neurons, neuronal stem cells lining the telencephalic lateral ventricles generate new interneurons of the olfactory bulb. The lifelong turnover of interneurons in the olfactory bulb occurs in parallel with a constant turnover of olfactory receptor neurons in the olfactory eptihelium. Recent work in our lab focuses on how the newly generated olfactory neurons face the challenge to integrate into the existing olfactory circuitry. Thereby, we are particularly interested in how axons of olfactory receptor neurons find their way to the correct target areas in the olfactory bulb, and how the formation of new synapses in glomeruli is regulated. We also investigate the mechanisms that regulate the turnover of bulbar interneurons.
Newly formed olfactory receptor neurons extend axons into the olfactory bulb, where they face the challenge to integrate into existing circuitry. The consensus view is that in vertebrates individual receptor neurons project unbranched axons into one specific glomerulus of the olfactory bulb. Axons of Xenopus olfactory receptor neurons bifurcate and innervate multiple glomeruli. Strikingly different from the generally assumed wiring principle in vertebrate olfactory systems, axons of single receptor neurons of Xenopus laevis regularly bifurcate, and project into more than one glomerulus (Nezlin and Schild, 2005; Hassenklöver and Manzini, 2013). Specifically, the innervation of multiple glomeruli is present in all ontogenetic stages of this species, from the larva to the post-metamorphic frog. Also, we show that this unexpected wiring pattern is not restricted to axons of immature receptor neurons, but that it is also a feature of mature neurons of both the main and accessory olfactory system (Hassenklöver and Manzini, 2013). This glomerular innervation pattern is unique among vertebrates investigated so far, and represents a new olfactory wiring strategy.