24 hr-survival after a single 3H-thymidine injection on E19 (medial sagittal)
Figure 107. This medial sagittal slice through the olfactory bulb on E20 slices the olfactory recess in several places where cells are spilling into the ventricle from the Olfactory NEP. There is diluted label after a 3H-thymidine exposure on the morning of E19 in the Olfactory NEP and SVZ, indicating active proliferation. Any unlabeled cells in the parenchyma were generated before the morning of E19. The few labeled cells in the parenchyma 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 in a compact clump. The formative AOB granule cell layer is outside the olfactory SVZ. In the MOB, the mitral cell layer contains densely-packed cells on the outer perimeter of the internal plexiform layer; the external plexiform layer is full of tufted cells, some are labeled. A thin granule cell layer may be forming outside the olfactory SVZ but it is quite indefinite and blends with the internal plexiform layer. 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 compared to the nerve layer. The presumptive olfactory epithelium is actively proliferating and has heavy label uptake in superficial and deep parts, as we noted in younger embryos.