24 hr-survival after a single 3H-thymidine injection on E18 (medial sagittal)
Figure 91. This medial sagittal slice through the most prominent part of the evaginating olfactory bulb on E19 shows heavy label uptake after a 3H-thymidine exposure on the morning of E18. Unlabeled cells were generated before the morning of E18. Most of the label uptake is in the neuroepithelium and subventricular zones in the core of the brain. The few labeled cells outside these layers are probably locally multiplying glia. The main and accessory parts of the olfactory bulb can be easily distinguished from each other. The AOB output neurons have settled in the posterodorsal part of the olfactory bulb but they do not form a compact clump because the slice is medial to that. For the first time, we can see a formative AOB granule cell layer outside the olfactory SVZ. In the MOB, the base of the mitral cell layer is clearly separated from the internal plexiform layer; the external plexiform layer is thicker than in E18 embryos. There is no evidence of a granule cell layer outside the olfactory SVZ; the broad layer at its superficial border is the internal plexiform layer, full of migrating cells generated on or before E17. The olfactory nerve is full of labeled glial cells; note the sharp decline in labeled cells in the outer edge of the external plexiform layer. The presumptive olfactory epithelium is actively proliferating and has heavy label uptake in superficial and deep parts, as we noted in younger embryos.