E11+12 to E21
By E21, the major cell groups in the developing olfactory bulb are recognizable. Using that survival time is helpful because it partially removes the problem of excessive label dilution. When consecutive daily injections are given to pregnant female rats on E11 and E12 (Figure 1) virtually every cell in the olfactory bulb is labeled on E21—many very lightly so that they may even appear to be unlabeled. Note that the group of output neurons of the accessory olfactory bulb (AOB) are more heavily labeled, and this population stands out in the slice. The more intense label indicates neurogenesis will occur sooner rather than later. Very few mitral cells are labeled with that intensity, indicating that they will be generated after the AOB output neurons. Occasional heavily labeled large cells are scattered in the plexiform layers of both the accessory and main (MOB) bulbs. The external plexiform cells are the earliest generated tufted cells, and the internal plexiform cells are the earliest generated mitral cells that may still be migrating on E21. About 5% of the cells in each population are generated before E13.