Age-related changes in cell proliferation, neuronal differentiation, and cell death in mouse olfactory neuroepithelium were investigated. Mice at the age of 10 days through 16 months were given a single injection of bromodeoxyuridine (BrdU). The olfactory mucosae were fixed at 9 timepoints ranging from 2 hours to 3 months after the injection and examined using double immunostaining for BrdU and olfactory marker protein (OMP), and double staining with terminal deoxynucleotidyl transferase-mediated biotinylated dUTP nick end labeling (TUNEL) and immunostaining for OMP. The number of BrdU-labeled cells/mm epithelial length initially increased, peaked at 2-3 days after the BrdU injection, then declined at each age. The number of BrdU- and TUNEL-labeled neuronal cells both decreased with increasing age, suggesting that the rates of both cell proliferation and cell death in the olfactory neuroepithelium decrease with increasing age. Double-labeled cells for BrdU and OMP appeared at 7 days after injection in all age groups, suggesting that the time required for neuronal differentiation is broadly similar irrespective of age. In older age groups, smaller amounts of the newly produced cohort are integrated into the OMP-positive ORN population, and even once it is integrated it is eliminated from the population more rapidly compared to the younger age groups. Furthermore, TUNEL assay showed that the fraction of apoptotic cells distributed in the OMP-positive layer/total apoptotic cells decreased with age. This observation suggests that the turnover of mature ORNs is slower in the older neuroepithelium compared to the younger neuroepithelium.
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