Methacrylate-embedded E17 embryo (anterior coronal)

Figure 70.  This anterior coronal slice cuts through the main olfactory bulb of a normal embryo killed on the morning of E17.  An indistinct dorsal area may contain some differentiating cells that will be in the future AOB.  In the MOB, cells are beginning to sort in concentric layers: the olfactory NEP is the dense inner core, the less dense SVZ is difficult to distinguish from the deep part of 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 E17 autoradiographic specimens show label uptake.  The different locations of the mitotic phase of the cell cycle indicates two distinct sets of multiplying cells: one may be the sensory neurons, the other may be the supporting cells.  The sensory epithelium in Jacobson’s organ is actively proliferating, and bundles of vomeronasal nerve fibers are forming in a superficial location.  A few bundles of the vomeronasal nerve grow upward along either side of the nasal septum.