24 hr survival after a 3H-thymidine injection on E17 (medial sagittal)
Figure 74. This medial sagittal slice through the most prominent part of the evaginating olfactory bulb on E18 shows heavy label uptake after a 3H-thymidine exposure on the morning of E17. Unlabeled cells were generated before the morning of E17. The unlabeled cells in the parenchyma outside the olfactory NEP and SVZ are AOB output neurons and many (98%) mitral cells generated before E17. There is more definite lamination in the olfactory bulb itself, and an MOB can be distinguished from the AOB; the AOB output neurons are settled in a pod-like area in the posterodorsal part of the olfactory bulb. The mitral cell layer is more recognizable along the outer edges of the internal plexiform layer, and the external plexiform layer is thicker than in E17 embryos. The olfactory nerve is full of large cells, some unlabeled and many labeled that coalesce with the brain surface where the bulb continues to evaginate—much farther than E17 embryos. The presumptive olfactory epithelium is actively proliferating and has heavy label uptake. Note that even 24 hrs after a single 3H-thymidine exposure there is greater uptake in superficial and deep parts of the epithelium. The sensory epithelium in Jacobson’s organ shows the same pattern. Individual bundles of the vomeronasal nerve are growing toward the brain above Jacobson’s organ.