Development of P2 olfactory glomeruli in P2-internal ribosome entry site-tau-lacZ transgenic mice

Primary olfactory neurons project their axons to the olfactory bulb, where they terminate in discrete loci called glomeruli. All neurons expressing th e same odorant receptor appear to terminate in a few glomeruli in each olfa ctory bulb. In the P2-IRES-tau-LacZ line of transgenic mice, LacZ is expres sed in the perikarya and axons of primary olfactory neurons that express th e P2 odorant receptor. In the present study, we examined the developmental appearance of P2 neurons, the topographical targeting of P2 axons, as well as the formation of P2 glomeruli in the olfactory bulb. P2 axons were first detected in the olfactory nerve fiber layer at embryonic day 14.5 (E14.5), and by E15.5 these axons terminated in a broad locus in the presumptive gl omerular layer. During the next 5 embryonic days, the elongated cluster of axons developed into discrete glomerulus-like structures. In many cases, gl omeruli appeared as pairs, which were initially connected by a fascicle of P2 axons. This connection was lost by postnatal day 7.5, and double glomeru li at the same locus were observed in 85% of adult animals. During the earl y postnatal period, there was considerable mistargeting of P2 axons. In som e cases P2 axons entered inappropriate glomeruli or continued to grow past the glomerular layer into the deeper layers of the olfactory bulb. These ab errant axons were not observed in adult animals. These results indicate tha t olfactory axons exhibit errors while converging onto a specific glomerulu s and suggest that guidance cues may be diffusely distributed at target sit es in the olfactory bulb.