24 hr survival after a single 3H-thymidine injection on E20 (anterior coronal)
Figure 127 is an anterior coronal slice through the MOB in an E21 embryo killed 24 hrs after a single exposure to 3H-thymidine on E20; note the NEP cells oozing into the ventricular lumen of the olfactory recess on the right side—typical of other E21 specimens. Labeled cells are present mainly in the olfactory NEP, the SVZ, and the olfactory nerve layer. Nearly all cells are unlabeled in the internal and external plexiform layers and the mitral cell layer—all generated before the morning of E20. The few labeled cells are probably locally-multiplying glia or rapidly migrating interstitial tufted cells. The coronal section plane always shows the best definition of layers in the main olfactory bulb. The beginnings of a granule cell layer are the densely-packed cells outside the olfactory SVZ that blends with the deep part of the internal plexiform layer. The sharp decline in labeled cells in the external plexiform layer indicates the glia in the olfactory nerve “know” the outer limits of the brain. The heavy label uptake in the olfactory epithelium indicates active proliferation, some parts show a separation between superficial and deep labeled cells, but other areas have homogeneous label uptake.