Methacrylate-embedded E18 embryo (anterior coronal)
Figure 87. This anterior coronal slice cuts through the main olfactory bulb of a normal embryo killed on the morning of E18. Cells are sorted in concentric layers around the lumen of the olfactory recess of the lateral ventricle: the olfactory NEP is the dense innermost layer, the less dense SVZ is difficult to distinguish from the deep part of the internal plexiform layer, spindle-shaped cells fill the internal plexiform layer, large cells with dense cytoplasm are clumping together along the outer edge of the internal plexiform layer, outside of that is the fibrous and cell-sparse external plexiform layer, finally the olfactory nerve layer is full of cells and fibers. Many bundles of the olfactory nerve are along the superficial edges of the olfactory epithelium, some are continuous with fibers coming out of the epithelium. In the epithelium itself, there are superficial and deep dividing cells—just where the short-survival E18 autoradiographic specimens show label uptake—possibly two distinct sets of multiplying cells. Olfactory sensory neurons may be the large cells in the superficial epithelium. The sensory epithelium in Jacobson’s organ is actively proliferating, and bundles of vomeronasal nerve fibers are forming in a superficial location. Several bundles of the vomeronasal nerve grow upward along either side of the nasal septum.