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3H-thymidine injections from E16 to E17 and survival to postnatal day (P)5 (anterior coronal)

Figure 42.  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.  Between E21 and P5, nearly all migrating mitral cells are settled.  Also, the labeled cells are scattered throughout the depth of the mitral cell layer, not just in the deep parts.  The olfactory bulb is growing in length and circumference, and the mitral cell layer is being stretched out into a thinner layer.  Thus, the older superficial to younger deep neurogenetic gradient that we see on E21 disappears by P5.  Large cells in the external plexiform layer are tufted cells. Many large cells close to the mitral cell layer are unlabeled; the amount of labeling increases as one goes superficially.  Thus, tufted cells do show a neurogenetic gradient in the mature MOB: deep cells are older than superficial cells (Bayer, 1983, Exp Brain Res. 50:329-340).  Most of the small cells in the granular layer, the internal and external plexiform layers, and the glomerular/olfactory nerve layers are labeled.  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 into the olfactory bulb from the proliferating subependymal layer surrounding the olfactory recess and anterior horn of the lateral ventricle.