Methacrylate-embedded E21 embryo in the horizontal plane (dorsal)
Figure 131 is a horizontal slice that grazes the dorsal surfaces of the accessory and main parts of the olfactory bulb in an E21 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 in the MOB internal plexiform layer. 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.