Methacrylate-embedded E20 embryo in the sagittal plane (medial)
Figure 114. This medial sagittal slice cuts through the olfactory recess of the lateral ventricle, olfactory NEP, and olfactory SVZ. Cells oozing into the ventricle are prominent in the recess. The AOB is in the posterodorsal part of the bulb, but does not form a prominent “hump” medially. The borders of the mitral cell layer in the MOB are becoming more distinct as fewer clusters of cells move in from the internal plexiform layer. There are widely-spaced tufted cells among the fibers in the expanded external plexiform layer. Many fibers are crisscrossing in the olfactory nerve/glomerular layers. Spindle-shaped cells appear to be migrating into the internal plexiform layer in the ventral MOB near the anterior part of the anterior olfactory nucleus. 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 that is now composed of cartilage; the bone is growing around the already established olfactory nerve bundles. The superficial half of the olfactory epithelium is composed of large olfactory sensory neurons with darkly-staining cytoplasm.