Methacrylate-embedded E22 embryo in the sagittal plane (lateral)
Figure 137. This lateral sagittal slice, from the same specimen in Figure 136 probably cuts the olfactory recess where there is a large clump of cells oozing into the ventricular lumen; that completely obliterates what is left of the olfactory recess. The accessory olfactory bulb is larger here, with densely-packed output neurons sandwiched between the external and internal plexiform layers on the posterodorsal surface. The thick internal plexiform layer is contiguous with the same layer in the MOB, and cells migrating through this layer may bypass the AOB and go directly into the MOB. There is a distinct cluster of densely-packed AOB granule cells below the internal plexiform layer that has a distinctive arrangement of embedded fibers. The cells that have a different packing density between the MOB and the AOB may be the pars externa of the anterior olfactory nucleus. In the MOB proper, dense cells in olfactory SVZ form the core. The internal plexiform layer has large cells that are either migrating tangential to the mitral cell layer or are turned radially as if to join the mitral cells. Most mitral cells have a buildup of densely staining cytoplasm—no doubt these materials will be spun out into the characteristic mitral cell dendritic arbors in the glomeruli. A fibrous zone—the external plexiform layer—lies in between the outer mitral cells and the olfactory nerve layer. Many small bundles of olfactory nerve fibers exit the olfactory epithelium and coalesce into a thick ventral olfactory nerve layer that thins as it continues to the dorsal surface of the bulb. The epithelium itself is filled with large superficial cells that have densely-staining cytoplasm; these are probably olfactory sensory neurons growing axons into the olfactory nerve. There are many more sensory neurons than in earlier age groups, taking up nearly the entire depth of the layer.