3H-thymidine injections from E15 to E16 and survival to postnatal day (P)5 (anterior)

Figure 26 slices only through the main olfactory bulb.  On the morning of E15, most main bulb mitral cells are labeled; several are unlabeled, but they are not always in a superficial position.  Between E21 and P5 the mitral cell layer occupies a larger circumference and a much longer anteroposterior space in the forebrain.  That growth pattern stretches out the mitral cell layer so that the older superficial mitral cells generated before E15 and still superficial on E21 are now adjacent to labeled cells that were generated on or after E15 by P5.  Approximately 20% of the mitral cell population is generated on or before E14 (Bayer, 1983, Exp. Br. Res. 30:329-340). Note the lack of heavily labeled cells in the internal plexiform layer compared to the E21 specimen in Figure 25; that indicates that few to no mitral cells are still migrating on P5.  The granule cells themselves are lightly labeled with extensive label dilution; they will be generated much later.  The immature cells in the core of the olfactory bulb and in the presumptive rostral migratory stream all appear to be unlabeled, but that is because of extensive label dilution.  Thus, on the morning of E15, most main bulb neuronal populations are still proliferating in their respective germinal matrices.