Quantification of the effects of long-term unilateral naris closure on the olfactory bulbs of adult mice.

The effects of unilateral naris closure on the olfactory bulbs of adult mice were assessed quantitatively by measuring four parameters. Naris closures were performed when animals were at least 5 months of age and lasted for 4-8 months. The first parameter measured was mitral cell number, which revealed that there was no significant effect of closure on numbers of these cells. The next parameter measured was the area of the external plexiform layer (EPL). The area of the EPL was 41% smaller in the closed-side olfactory bulbs than in the open-side olfactory bulbs (P less than 0.01). Comparisons of the areas of the granule cell layers (GCL) showed that the closed-side GCL was 25% smaller than the open-side GCL (P less than 0.01). Finally, the number of cells in the open- and closed-side GCLs were compared. The number of cells in the closed-side GCL was 30% lower than the number of cells in the open-side GCL (P less than 0.01). These data show that the shrinkage of closed-side olfactory bulb after naris closure in adult mice is due, at least in part, to the loss of granule cells and not to the loss of the main output neurons, the mitral cells.