Effects of unilateral olfactory deprivation in the developing opossum, Monodelphis domestica.

Unilateral naris closure in young rodents leads to striking alterations in the development of the ipsilateral olfactory system. One of the most pronounced effects is a 25% reduction in the size of the experimental olfactory bulb, a change that stems in part from decreased cell survival. Since naris occlusion in rodents alters the system more during development than in adulthood, we investigated the consequences of olfactory deprivation in a species that is born in a very immature state, Monodelphis domestica. In this pouchless marsupial, offspring are born after a short 14-day gestation. In the present study, the thymidine analogue bromodeoxyuridine was used to examine early postnatal neurogenesis in the olfactory bulb. Unlike rats and mice, neurogenesis of the main output neurons (the mitral cells) continues into postnatal life. Unilateral naris closure was begun on postnatal day 4 (P4) or P5 in Monodelphis and continued for 30 or 60 days. Laminar volume measurements revealed a significant reduction in the size of the experimental bulb following 60, but not 30, days of early olfactory deprivation. Mitral cell number estimates indicated a significant reduction after both 30 and 60 days of naris closure. The immaturity of Monodelphis offspring may render the population of mitral cells susceptible to the effects of olfactory deprivation. These findings suggest that afferent activity plays a role in the survival of all bulb neurons, irrespective of cell class.