Supporting cell proliferation in the olfactory epithelium decreases postnatally.

It is well known that progenitor cells in the basal layer of olfactory epithelium proliferate continuously throughout life; the offspring of these dividing cells produce replacements for receptor neurons. In the rat the number of proliferating basal cells/mm length of epithelium (proliferation density) decreases with postnatal age while the area of the olfactory sheet increases. The supporting cells, which act as the glia of the olfactory epithelium, also divide. We examined in detail some aspects of the dynamics of olfactory supporting cell proliferation to determine whether their rate of proliferation changes with age, and how it compares with the rate in basal progenitor cells. Using BrdU to label dividing cells, we determined the proliferation density of supporting cells and basal cells in 10 microm coronal sections from six different anterior-posterior regions in rats ranging in age from birth (P1) until P333. We observed a dramatic decrease in supporting cell proliferation density from P1 (80 cells/mm) to P11 (32 cells/mm) to P21 (12 cells/mm); the density decreases continuously to P333 (0.4 cells/mm). This reduction was even more dramatic than that in the basal cell population (Weiler and Farbman, 1997). Analysis of the data for correlation between basal and supporting cell proliferation revealed a weak correlation in neonates but no correlation in older animals. This suggests that the mechanisms that regulate proliferation of the two cell types are different. Our data also indicate that the proliferation of supporting cells is related only to growth in surface area of the epithelium. No turnover seems to occur in the supporting cells as it does in the olfactory neurons, where proliferation of basal cells is necessary for both growth and replacement.