2 hr survival after a 3H-thymidine injection on E17 (sagittal)
Figure 63. A sagittal slice through the olfactory bulb and nasal cavity in an E17 embryo exposed to 3H-thymidine 2 hrs before death. The superficial parts of the NEP in all areas are filled with proliferating cells in the S-phase of the mitotic cycle. The forebrain NEP is cut tangentially, making it appear very thick. Differentiating neurons outside the NEP are expanding the parenchyma, especially in the region of the basal ganglia, the basal telencephalon, and the evaginating olfactory bulb. There is very little laminar differentiation in the olfactory bulb, but a small group of unlabeled cells on the dorsal surface of the bulb may be the AOB output neurons. There is a sharp separation between the outermost unlabeled cells of the brain parenchyma and the heavy label uptake in the olfactory nerve. The labeled cells in the nerve are locally multiplying glia that nurture the olfactory nerve fibers. The olfactory epithelium is actively proliferating; note that label uptake is heavy bordering the lumen of the nasal cavity and at the superficial border of the epithelium. These two different regions of label uptake indicate two different populations of proliferating cells.