Distribution of Notch1-expressing cells and proliferating cells in mouse vomeronasal organ.

Vomeronasal receptor neurons (VRNs) proliferate and differentiate continuously in the vomeronasal organ (VNO) throughout life. In adult mice, new VRNs are generated mainly in the marginal region, located in the boundary region between sensory and nonsensory epithelia. The Notch signaling pathway is involved in differentiation in the developing nervous system. To understand the Notch signaling pathway involved in generating VRNs, we focused on the relationship between the expression pattern of Notch1 and the localization of proliferating cells in both developing and regenerating mice VNO, and examined the Notch signaling pathway involved in the development of VNO by in situ hybridization of Notch1 and immunocytochemistry of 5-bromo-2'-deoxyuridine. During embryonic and neonatal development, proliferating cells and Notch1-expressing (+) cells were observed evenly throughout VNO. A large number of proliferating cells and Notch1 (+) cells were observed in embryonic VNO, but gradually decreased during development. The localization of proliferating cells was similar to that of Notch1 (+) cells at each developmental stage. In adult VNO, there are a few proliferating cells and Notch1 (+) cells, which were only in the marginal region of VNO. Seven days after removal of the accessory olfactory bulb (AOB), VRNs proliferated throughout VNO. Although the number of Notch1 (+) cells also increased in VNO, the majority of these were concentrated in the dorsal region of VNO, suggesting that it has two types of differentiating cell. These results suggest that Notch1 plays a role in the differentiation of VRNs during development and regeneration of VRNs after removal of AOB.