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Methacrylate-embedded E22 embryo in the coronal plane (posterior)

Figure 142.  This is a slice from the same specimen in Figure 141 cut farther back into the olfactory peduncle where the accessory olfactory bulb occupies the dorsal surface.  The AOB output neurons—much smaller than mitral cells—form a well-defined clump sandwiched between the external and internal plexiform layers.  The granule cells of the AOB are settling deep to the internal plexiform layer and are more easily distinguished from the MOB granule cells because of a distinctive layer of embedded fibers.  In the MOB, the layers are nearly the same as in Figure 141.  The granule cell layer is indistinguishable from the olfactory SVZ, that is just outside the columnar cells of the olfactory NEP/ependyma.  The pars externa of the anterior olfactory nucleus may be on either side of the AOB, but the rest of the nucleus is farther back in the peduncle and not in this slice.  Just as in Figure 141, the mitral cell layer forms a continuous U-shaped layer beneath the AOB part of the slice.  Note the many smaller cells at the base of the mitral cell layer–these are the granule cells.  Other granule cells are in the internal plexiform layer and will infiltrate the mitral cells.