Methacrylate-embedded E18 embryo (dorsal horizontal)
Figure 84 is the dorsal horizontal slice through the accessory part of the olfactory bulb in an E18 specimen. The slice cuts through the ventral limits of the cortical plate laterally and through the transitional NEPs of the neocortex, septum, and striatum; the slice is above the olfactory NEP. The horizontal cutting plane is always the best to show migration streams, and many spindle-shaped cells appear to be migrating toward the settling AOB output neurons (curved solid-line arrows). The dashed-line arrow traces a more tentative migratory stream coming into the AOB from a deep position anterior to the tenia tecta. The double-ended arrow traces a prominent migratory stream that forms a cap over the anterior extent of the presumptive cortical SVZ; these cells may be migrating in both directions—lateral to medial and medial to lateral. A narrow zone between the cortical and striatal NEPs shows a prominent exodus of spindle shaped cells penetrating fibers, possibly heading toward the stratified transitional field in the neocortex. Of course, it is possible that some of these migrating cells may be mitral cells that will settle in the posteromedial parts of the MOB.