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Methacrylate-embedded E22 embryo in the sagittal plane (medial)

Figure 136.  This medial sagittal slice cuts through the dense olfactory ependyma and SVZ continuous with the basal telencephalic ependyma and SVZ (the olfactory recess is lateral to the slice) and generally has similar features to the E21 morphology. The AOB in the posterodorsal part of the bulb does not form a prominent “hump,” because it is lateral to this slice.  The internal border of the mitral cell layer in the MOB is distinct as fewer clusters of cells move in from the internal plexiform layer.  The internal plexiform layer is reduced to a narrow cell-sparse zone on the perimeter of the formative granule cell layer.  There are widely-spaced tufted cells among the fibers in the external plexiform layer.  Many fibers are crisscrossing in the olfactory nerve/glomerular layers.  Very few cells are migrating into the internal plexiform layer in the ventral MOB near the anterior olfactory nucleus—mitral cell migration is nearly completed.  Coalesced fiber bundles of the olfactory nerve surround the bulb and form a thick ventral bundle.  Individual bundles of the olfactory nerve penetrate the undulations of the cribriform plate of the ethmoid bone; the bone is growing around the olfactory nerve bundles producing tiny pinhole foramina.  Most of the olfactory epithelium is composed of large olfactory sensory neurons with darkly-staining cytoplasm.