Cell surface carbohydrates have been implicated in axon guidance and targeting throughout the nervous system. We have begun to test the hypothesis that, in the olfactory system, a differential distribution of cell surface carbohydrates may influence olfactory sensory neuron (OSN) axon targeting. Specifically, we have examined the spatial distribution of two different plant lectins, Ulex europaeus agglutinin (UEA) and Dolichos biflorus agglutinin (DBA), to determine whether they exhibit differential and reproducible projections onto the main olfactory bulb. Each lectin exhibited a unique spatial domain of glomerular labeling that was consistent across animals. UEA labeling was strongest in the ventral aspect of the olfactory bulb; DBA labeling was strongest in the dorsal aspect of the olfactory bulb. Some evidence for colocalization was present where these two borders intersected. Large areas of the glomerular layer were not labeled by either lectin. To determine whether patterns of lectin labeling were reproducible at the level of individual glomeruli, UEA labeling was assessed relative to M72-IRES-taulacZ- and P2-IRES-taulacZ-labeled axons. Although glomeruli neighboring these two identified glomeruli were consistently labeled with UEA, none of the lacZ positive axons was lectin labeled. Labeling of vomeronasal sensory neuron axons in the accessory olfactory bulb was more uniform for the two lectins. These data are the first to show a differential distribution of UEA vs. DBA labeling in the main olfactory bulb and are consistent with the hypothesis that a differential distribution of cell surface carbohydrates, a glycocode, may contribute to the targeting of OSN axons.
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.
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.
Preauricular swelling is a common presenting symptom for patients visiting an otolaryngologist. Often this symptom is suggestive of a parotid mass. However, lesions of the temporomandibular joint and mandible may also present in this fashion.
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