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Methacrylate-embedded E19 embryo (lateral sagittal)

Figure 99.  This sagittal slice, from the same specimen in Figure 98, is lateral to 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 dorsal 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, the dense core is a tangential cut of NEP cells.  Cells in the olfactory SVZ surround the NEP; there is an indistinct border between these two layers.  The superficial parts of the internal plexiform layer have clumps of larger cells that will join the mitral cell layer.  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.  Mitotic figures are less common than in earlier ages, but most are either deep or superficial—just like the short-survival E19 autoradiography pattern.