24 hr-survival after a single 3H-thymidine injection on E19 (anterior coronal)
Figure 109 is an anterior coronal slice through the MOB in an E20 embryo killed 24 hrs after a single exposure to 3H-thymidine on E19; note the NEP cells oozing into the ventricular lumen of the olfactory recess—typical of other E20 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 E19. The few labeled cells are probably locally-multiplying glia or rapidly migrating tufted cells. The coronal section plane always shows the best definition of layers in the main olfactory bulb. This plane shows the beginnings of a granule cell layer as densely-packed cells outside the olfactory SVZ. The olfactory nerve blends with the ventral brain surface in the contact zone, obliterating the pial membrane, but the sharp decline in labeled cells in the external plexiform layer indicates the peripheral glia “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.