Analysis of molecular mechanisms for neurogenesis in the mammalian olfactory sensory epithelium

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Neurons are continuously generated in the mammalian olfactory sensory epithelium even in the adult period. To examine the cellular and molecular mechanism of adult neurogensis, neurogenesis in the olfactory epithelium was enhanced by the unilateral removal of the olfactory bulb (bulbeclomy) in the adult mice, and the newly-generated neurons were followed by bromodeoxyuridine (BrdU) labeling. Because the enhancement of neurogenesis was more prominent in the vomeronasal organ (VNO) specified for pheromone sensation rather than in the main olfactory epithelium for odorant detection, VNO was intensively examined in this study. In the VNO of control side, BrdU labeled ceils were sparsely located at the basal layer. In contrast, in the operated side, many BrdU labeled cells appeared throughout the basal layer, which indicated that most stem cells are quiescent but can be stimulated to proliferate with bulbectomy. Within several days BrdU labeled cells migrated to superficial layers and expressed neuronal marker proteins. There are two subtypes in the sensory neurons in VNO, the Go positive and the Gi positive neurons. BrdU labeling experiments showed that new neurons were at first Go positive immature cells and then differentiated into Go positive and Gi positive neurons, indicating that two types of neurons in the VNO originate from common Go positive progenitors. To analyze the molecular mechanism of olfactory neurogenesis in detail, nestin promoter-EGFP transgenic mice were utilized, where newly-generated cells can be visualized by GFP fluorescence. GFP expressing cells in VNO were found to be immature progenitors from their marker protein expression. The sensory epithelium of the transgenic mice was stripped, the cells were dispersed, and the GFP expressing cells were successfully recovered under fluorescent microscopy. This system was considered to be useful to further examine the molecular mechanism of olfactory neurogenesis in vitro.