Neurogenesis, migration, and apoptosis in the vomeronasal epithelium of adult mice.

The location of neurogenesis and the direction of migration of neurons in the adult mouse vomeronasal organ is controversial. Cell division occurs at the center, and particularly, at the edges of the epithelium. Newly generated cells at the center of the epithelium participate in neurogenesis, however, it is unknown to what extent dividing cells at the edges participate in growth, become apoptotic or mature into neurons. Premitotic cells were labeled with bromodeoxyuridine (BrdU) in adult mice and animals allowed to survive for different postinjection periods. The terminal deoxynucleotidyl transferase-mediated biotinylated dUTP nick end-labeling (TUNEL) method was used to show the distribution of apoptotic cells. The vertical and horizontal position of BrdU-labeled cells was analyzed as a function of postinjection survival time. Vertical and horizontal migration of BrdU-labeled cells were detected. Cells in the central portions of the epithelium migrated vertically to become neurons as demonstrated by co-expression of olfactory marker protein. Cells at the edges migrated horizontally very slowly (less than 10% of the distance from the edge to the center of the epithelium per month), thus indicating that these cells participate in cell renewal exclusively in marginal regions. Neural turnover in the mouse vomeronasal epithelium, therefore appears to occur through a process of vertical migration. Data on the distribution of apoptotic cells indicate that a number of dividing cells throughout the epithelium, but particularly at the edges, die before becoming functional neurons. Accordingly, most dividing cells at the edges probably constitute a reservoir of stem cells dying before differentiation.