Postnatal development of olfactory receptor cell axonal arbors.

The mechanisms that subserve the distribution of the terminal arbors of olfactory receptor cell axons remain unknown. Elsewhere in the central 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 arbors on postnatal days 0, 3, 6, 9, 12, and 21. The olfactory bulbs from Sprague-Dawley rats were processed using a Golgi technique that impregnated ORN axons. Axons from each age group were reconstructed by using camera lucida at x100, oil immersion, and morphometrically characterized. In the adult, the percent glomerular area occupied by a single ORN axon was 14%, whereas the mean length of branches was 169.67 microm, the sum of branches and varicosities was 27, and the distance to first branch point in glomeruli was 21.98 microm. The values from the younger age groups were not statistically different from those in the adult. Because there was no evidence of early exuberance, our data suggest that ORN axons must innervate single glomeruli and arborize in a highly specific manner to achieve the adult pattern. Because our data suggest that ORN axons do not follow the hypothesized scheme, it is plausible to suggest that as ORN axons innervate a glomerulus during development they arborize to their adult levels but not beyond. This argues strongly that specific cell surface and trophic factors are used by the ORN axon to guide glomerular targeting and innervation.