Methacrylate-embedded E17 embryo (dorsal horizontal)
Figure 67 is the dorsal horizontal slice through the olfactory bulb in an E17 specimen. The horizontal cutting plane best shows the migration streams coming into the bulb from the basal telencephalon. This dorsal slice cuts through the part of the bulb that will differentiate into the AOB. Straight arrows indicate radial migration from the olfactory and basal telencephalic NEPs into the enlarging brain parenchyma. On the medial side, dashed-line arrows trace cells migrating toward the AOB from a medial germinal source near the septal NEP. In the AOB, the outermost layer is the vomeronasal/glomerular layer, next is the very thin external plexiform layer, the settling output neurons, and a thick internal plexiform layer that is full of spindle-shaped cells migrating around the forward tip of the olfactory NEP and SVZ; this layer extends without a break into the MOB on the lateral side of the bulb. Note the thick accumulation of fibers in the lateral olfactory tract and how the spindle-shaped cells radiate outward into the internal plexiform layer (solid-line curved arrows). Some of these cells may be AOB output neurons as well, but our suspicion is that most of AOB neurons take the medial migratory route. This slice also cuts the primary olfactory cortex where three layers can be distinguished. The anterior olfactory nucleus may be appearing between the fibers of the anterior commissure and the lateral olfactory tract.