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Methacrylate-embedded E22 embryo in the horizontal plane (dorsal)

Figure 138 is a horizontal slice that grazes the dorsal surfaces of the accessory and main parts of the olfactory bulb in an E22 specimen.  The tangential cut makes the layers appear thick.  The smaller AOB output neurons are easily distinguished from the larger MOB mitral cells with their densely-staining cytoplasm.  The horizontal cutting plane is always the best to show migration streams, and many spindle-shaped cells are still in the MOB internal plexiform layer—though many fewer than in earlier age groups.  The discontinuity between the AOB and MOB internal plexiform layers may be the pars externa of the anterior olfactory nucleus.  The AOB granule cells are tentatively identified in front of the internal plexiform layer—they blend imperceptibly with the MOB formative granule cell layer; one of the identifying features is the distinctive layer of fibers among the AOB granule cells.  But in the MOB, the forming granule cell layer shows a distinct maturation gradient: superficial cells are larger and more differentiated than deep cells (see red-outlined clusters).