24 hr survival after a single 3H-thymidine injection on E20 (medial sagittal)
Figure 125. This medial sagittal slice through the olfactory bulb on E21 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 E20 in the Olfactory NEP and SVZ, indicating active proliferation—most cells are labeled. Any unlabeled cells in the parenchyma were generated before the morning of E20. The few labeled cells in the parenchyma are probably locally multiplying glia or endothelial cells. 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 with a few heavily labeled cells that were generated on E20. In the MOB, the mitral cell layer contains densely-packed cells (nearly all unlabeled) on the outer perimeter of the internal plexiform layer; the external plexiform layer is full of tufted cells (also unlabeled). A thin granule cell layer may be forming in the deep part of 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 some parts, there are clumps of superficial and deep uptake, while other parts are more evenly labeled.