Proliferation in the Olfactory Epithelium: Changes During Postnatal Life in the Rat

Vertebrate olfactory sensory neurons are continuously replaced throughout life by the division of basal cells and differentiation of the daughter cells into neurons. In contrast with other mammals, the rat's olfactory receptor sheet continues to grow and increase in size even in the adult (Hinds and McNelly, 1981). The question arises: Is there an increase in proliferation because of the increased number of neurons that have to be replaced? We used the BrdU-method to label dividing cells in the olfactory epithelium, and counted proliferating cells in 10 (im paraffin sections of male and female Sprague-Dawley rats aged PI (birth) to P333. The proliferation density number of labeled cells per mm olfactory epithelium = 'cells' in able) in the basal compartment decreases dramatically and continuously with age: Thus, although there is a pronounced increase in total number of neurons, the density of proliferating cells decreases with age. With the increase in size of the olfactory sheet the expectation is that the number of proliferating cells would also increase, to replace dying cells in a larger neuronal population, assuming that the life span of individual neurons did not change. Our results indicate that the life span of mature cells must increase in the older animals. This finding is consistent with those in an earlier report, that the relative number of mature neurons increases with age whereas the number of immature neurons decreases (Verhaagen et a/.,1989,J. Neurosci., 9:683)