POSTNATAL-DEVELOPMENT OF OLFACTORY RECEPTOR CELL AXONAL ARBORS

The mechanisms that subserve the distribution of the terminal arbors o f olfactory receptor cell axons remain unknown. Elsewhere in the centr al nervous system, a common theme is early axonal exuberance followed by activity-dependent pruning to achieve the mature distribution. This led to the hypothesis that-the orderly morphology of afferent; axons in the olfactory glomerulus may follow a similar developmental scheme of exuberance followed by pruning. To test this hypothesis, we studied morphological features of olfactory receptor neuron (ORN) axonal arbo rs on postnatal days 0, 3, 6, 9, 12, and 21. The olfactory bulbs from Sprague-Dawley rats were processed using a Golgi technique that impreg nated ORN axons. Axons from each age group were reconstructed by using camera lucida at x 100, oil immersion, and morphometrically character ized. In the adult, the percent glomerular area occupied by a single O RN axon was 14%, whereas the mean length of branches was 169.67 um, th e sum of branches and varicosities was 27, and the distance to first b ranch point in glomeruli was 21.98 mu m. The values from the younger a ge groups were not statistically different from those in the adult. Be cause there was no evidence of early exuberance, our data suggest that ORN axons must innervate single glomeruli and arborize in a highly sp ecific manner to achieve the adult pattern. Because our data suggest t hat ORN axons do not follow the hypothesized scheme, it is plausible t o suggest that as ORN axons innervate a glomerulus during development they arborize to their adult levels but not beyond. This argues strong ly that specific cell surface and trophic factors are used by the ORN axon to guide glomerular targeting and innervation. (C) 1998 Wiley-Lis s, Inc.