2 hr survival after a 3H-thymidine injection on E16 (sagittal)
Figure 46. A sagittal slice through the olfactory bulb and nerve in an E16 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 few cells outside the NEP are in the various subventricular zones—the most active zone is in the basal ganglionic eminence. 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 no laminar differentiation in these regions, so identifications are “educated guesses” about neuronal identity. 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—probably glia—that nurture the olfactory nerve fibers. The olfactory epithelium and Jacobson’s organ are actively proliferating.