Injection on E14, 2 hr survival
Figure 16 shows a sagittal-slice of an E14 embryo 2 hrs after its mother was injected with 3H-thymidine. There is heavy label uptake in superficial parts of all NEP areas and in the core of the basal ganglionic eminence—that is the prominent subventricular zone that is already well established by E14. Most NEP areas are proliferating to add to their numbers in the “stockbuilding phase” of development for future neurogenesis. The unlabeled cells outside the presumptive olfactory NEP are most likely the AOB output neurons that were generated on E12 (~60%). But outside the neocortical NEP you can see a thin layer of unlabeled cells—no doubt the Cajal-Retzius neurons generated on E12 and E13. The olfactory epithelium has a slit-like lumen in the snout and contains many proliferating cells. Many of the cells outside the brain are actively proliferating and the brain surface is hard to distinguish—especially in the basal forebrain. There is ample opportunity for cellular exchanges between the brain and periphery at these sites.