E13 injection-2 hr survival
Figure 7 shows a sagittal-slice of an E13 embryo 2 hrs after its mother was injected with 3H-thymidine. There is heavy label uptake in superficial parts of all neuroepithelial areas. The superficial position is where the S-phase of cell division occurs; mitosis occurs at the ventricular lumen. Note the high concentration of heavily labeled cells in the superficial olfactory bulb and basal ganglionic NEPS. We postulate that neurons generated on the day of an injection may be “sequestered” in the superficial NEP prior to moving out on the next day. There are several groups of unlabeled cells outside the NEP. From the E12 to E13 specimen shown in Figure 6, we can safely say that these cells (postmitotic neurons) were generated on E12. We know that many of the magnocellular basal telencephalic nuclei have substantial neurogenesis on E12 (Bayer, 1985, Int J Dev Neurosci 3:229-243), which helps to identify the large group of unlabeled cells outside the basal ganglionic NEP; most of these neurons will settle in the entopeduncular nucleus, posterior parts of the globus pallidus, the substantia innominata, and the horizontal limb of the diagonal band nucleus. The few unlabeled cells outside the presumptive olfactory NEP are most likely the few AOB output neurons that were generated on E12 (~8-9%). The other NEPS are proliferating to add to their numbers in the “stockbuilding phase” of development—neurogenesis will come later. But outside the neocortical NEP you can see a few large, unlabeled cells—no doubt the oldest Cajal-Retzius neurons generated on E12.
On E13, the olfactory epithelium is invaginating and there are some heavily-labeled cells both within it and without. The heavily labeled cells outside are most likely specialized mesenchyme cells that may be associated with fasciculating the olfactory nerve fibers as they grow toward the brain. Note that many of these cells absorbed 3H‑thymidine within 2 hrs, indicating that these cells are generated in situ—the pattern for glial and supporting cell proliferation. Some of these specialized cells may be contacting the superficial telencephalon in the area where the olfactory bulb evaginates. Note there is no part of the ventrobasal telencephalon that can be identified on E13 as the place where the olfactory NEP will evaginate, but the position of the specialized mesenchyme touching the basal telencephalon gives us a clue.