The olfactory epithelium and its neuronal population are known to have a substantial capacity to recover after either direct injury or damage to the olfactory nerve. However, the mechanisms underlying that capacity for recovery, and indeed the limits on the recovery process, are not well understood. The aim of this study is to describe in detail the way in which the olfactory epithelium reconstitutes after direct injury. Adult male rats were exposed to 330 ppm methyl bromide (MeBr) gas for a single 6-hour period. The exposure destroys all of the neurons and sustentacular cells in over 95% of the olfactory epithelium of food-restricted rats and in over 90% of the epithelium in ad-libitum-fed rats of the same weight, yet substantial recovery of the olfactory epithelium occurs. In response to the lesion, cellular proliferation increases markedly beginning between 24 and 48 hours, peaks at 1 week, and persists at levels higher than the control level for more than 4 weeks after MeBr exposure. Even though proliferation accelerates promptly, the beginning of neuronal reconstitution is delayed; only a few immature neurons are observed 3 days after the lesion, yet they reappear in large numbers by the end of the first week. The first mature neurons emerge between 7 and 14 days after lesion and increase to near normal numbers by 4-6 weeks. In association with the restoration of the neuronal population, basal cell proliferation returns to control levels between 4 and 6 weeks after damage. Likewise, sustentacular cells, identifiable by anticytokeratin 18 labeling, reappear rapidly and reform a distinct lamina in the superficial aspect of the epithelium. They closely resemble their counterparts in control epithelium with regard to disposition and shape by 3 weeks after lesion and with regard to expression of olfactory-specific cytochrome P450s by 8 weeks. Thus, most areas of the epithelium are restored to a near normal appearance and cellular composition by the end of 8 weeks, suggesting that the MeBr paradigm for lesioning the epithelium offers significant advantages over techniques such as Triton X-100 or ZnSO4 irrigation. However, not all measures of epithelial status are normal even at 8 weeks. Immature neurons remain slightly more numerous than normal at this time. Furthermore, some areas of the olfactory epithelium do not recover after MeBr lesion and are replaced by respiratory epithelium.(ABSTRACT TRUNCATED AT 400 WORDS)

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Glomeruli at the posterior margin of the main olfactory bulb differ in several respects from those located in the remainder of the bulb; e.g., the olfactory sensory neurons (OSNs) that project here exhibit a distinct biochemical phenotype and signal transduction pathway, the microcircuitry of the glomeruli is substantially altered, and the glomeruli are activated by unconventional odorants. In the present work, we report that the monoclonal antibodies 2C6 and MAb213 label distinct subsets of OSNs in the olfactory epithelium (OE), including their axons to their terminations in the main olfactory bulb (MOB). Neurons immunopositive with 2C6 are concentrated in the cul-de-sacs of ectoturbinates 1 and 2 and of endoturbinate IV. Unlike the vast majority of OSNs, 2C6(+) neurons express olfactory marker protein (OMP) at a low level, but their failure to stain with anti-GAP-43 labeling indicates that the OMP "weak" neurons are nonetheless mature. Glomeruli positive for 2C6 are bilaterally symmetrical and occupy reproducible positions along the posterior margin of the MOB. Three of these are very large, and we refer to them as the lateral, posterior ventral, and anterior ventral 2C6(+) necklace glomeruli. MAb213(+) neurons are concentrated in the posteriormost tips of the cul-de-sacs and recesses at the reflection of the OE at the cribriform plate. Like 2C6(+) neurons, MAb213(+) OSNs are weakly labeled with anti-OMP but are fully mature. MAb213(+) glomeruli are also bilaterally symmetrical; they occupy reproducible positions along the posterior margin of the MOB. The three largest glomeruli occupy lateral, posterior ventral, and posterior positions; the first two are found close to the aforementioned 2C6(+) glomeruli. MAb213 also intensely labels one of the glomeruli of the modified glomerular complex, a string of small glomeruli ventrally, and another string dorsal to the accessory olfactory bulb. Acetylcholinesterase (AChE) histochemical staining of adjacent sections showed that many, but not all, MAb213(+) glomeruli colocalize with dense or moderate AChE staining. Thus, it is likely that the "necklace olfactory glomeruli" (Shinoda et al., 1990, 1993) and the phosphodiesterase (PDE2)(+) glomeruli (Juilfs et al., 1997) are a subset(s) of the MAb213(+) glomeruli. On the other hand, 2C6(+) glomeruli are not associated with AChE staining. These data indicate that the 2C6(+) glomeruli comprise a novel subset in the posterior MOB. In addition to the 2C6(+) and MAb213(+) necklace glomeruli, there is another distinct set of glomeruli at the posterior margin of the bulb that are OMP(-), 2C6(-), and MAb213(-). In summary, the current work indicates that glomeruli at the posterior margin of the bulb, which are necklace glomeruli in terms of location and appearance, are actually heterogeneous and may subserve specialized functions within the olfactory system.

Glomeruli at the posterior margin of the main olfactory bulb differ in several respects from those located in the remainder of the bulb; e.g., the olfactory sensory neurons (OSNs) that project here exhibit a distinct biochemical phenotype and signal transduction pathway, the microcircuitry of the glomeruli is substantially altered, and the glomeruli are activated by unconventional odorants. In the present work, we report that the monoclonal antibodies 2C6 and MAb213 label distinct subsets of OSNs in the olfactory epithelium (OE), including their axons to their terminations in the main olfactory bulb (MOB). Neurons immunopositive with 2C6 are concentrated in the cul-de-sacs of ectoturbinates 1 and 2 and of endoturbinate IV. Unlike the vast majority of OSNs, 2C6(+) neurons express olfactory marker protein (OMP) at a low level, but their failure to stain with anti-GAP-43 labeling indicates that the OMP "weak" neurons are nonetheless mature. Glomeruli positive for 2C6 are bilaterally symmetrical and occupy reproducible positions along the posterior margin of the MOB. Three of these are very large, and we refer to them as the lateral, posterior ventral, and anterior ventral 2C6(+) necklace glomeruli. MAb213(+) neurons are concentrated in the posteriormost tips of the cul-de-sacs and recesses at the reflection of the OE at the cribriform plate. Like 2C6(+) neurons, MAb213(+) OSNs are weakly labeled with anti-OMP but are fully mature. MAb213(+) glomeruli are also bilaterally symmetrical; they occupy reproducible positions along the posterior margin of the MOB. The three largest glomeruli occupy lateral, posterior ventral, and posterior positions; the first two are found close to the aforementioned 2C6(+) glomeruli. MAb213 also intensely labels one of the glomeruli of the modified glomerular complex, a string of small glomeruli ventrally, and another string dorsal to the accessory olfactory bulb. Acetylcholinesterase (AChE) histochemical staining of adjacent sections showed that many, but not all, MAb213(+) glomeruli colocalize with dense or moderate AChE staining. Thus, it is likely that the "necklace olfactory glomeruli" (Shinoda et al., 1990, 1993) and the phosphodiesterase (PDE2)(+) glomeruli (Juilfs et al., 1997) are a subset(s) of the MAb213(+) glomeruli. On the other hand, 2C6(+) glomeruli are not associated with AChE staining. These data indicate that the 2C6(+) glomeruli comprise a novel subset in the posterior MOB. In addition to the 2C6(+) and MAb213(+) necklace glomeruli, there is another distinct set of glomeruli at the posterior margin of the bulb that are OMP(-), 2C6(-), and MAb213(-). In summary, the current work indicates that glomeruli at the posterior margin of the bulb, which are necklace glomeruli in terms of location and appearance, are actually heterogeneous and may subserve specialized functions within the olfactory system.