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3H-thymidine injections from E17 to E18 and survival to postnatal day (P)5 (anterior coronal)

Figure 58.  This slice is through all layers of the main olfactory bulb (MOB).  Compared to the E21 survival, there are more labeled cells in the mitral cell layer and only a few large cells are labeled in the internal plexiform layer.  By the morning of E17, nearly 90% of the mitral cells have been generated—look closely at the size of the labeled cells in the mitral cell layer and you will see that many of them are small; these are either endothelial cells (that can appear large depending how they lie in the slice) or multiplying glia.  As in the other P5 specimens, labeled mitral cells are scattered throughout the depth of the layer.  Many tufted cells in the deep half of the external plexiform layer are unlabeled, while large cells on the perimeters of the glomeruli (interstitial tufted cells) are nearly all labeled.  Most of the small cells in the granular layer, the internal and external plexiform layers, and the glomerular/olfactory nerve layers are very lightly labeled compared to the E21 survival—a consequence of label dilution.  The core of the MOB is full of small cells that are lightly labeled to unlabeled.  This area is the terminus of the rostral migratory stream that is a conduit of new neurons steadily migrating (arrows) into the olfactory bulb from the proliferating subependymal layer surrounding the olfactory recess and anterior horn of the lateral ventricle.