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Methacrylate-embedded E20 embryo in the sagittal plane (lateral)

Figure 115.  This sagittal slice, from the same specimen in Figure 114, is still cutting the olfactory recess and there is a large clump of cells oozing into the ventricular lumen.  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.  In the MOB proper, variously-oriented cells in the olfactory SVZ surround the columnar cells in the NEP; there is an indistinct border between these two layers.  The superficial parts of the internal plexiform layer have 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; this layer is much thicker than in E19 embryos.  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 approximately 2/3 to 3/4 of the depth of the layer.