Methacrylate-embedded E16 embryo (medial sagittal)

Figure 49.  This sagittal slice of the medial part of the olfactory bulb cuts the septum and the medial part of the basal ganglionic eminence.  There are many mitotic figures in the NEP at the ventricular lumen.  However, note that the mitotic figures are fewer in the olfactory recess.  The evaginating NEP may have different proliferative dynamics than NEPs in the neocortex or septum.  The superficial edges of the NEP in most parts of the forebrain are hard to delineate because many cells are migrating outward.  Once outside the NEP, the cells lose their columnar orientation.  Scattered mitotic figures outside of the presumptive NEP edges are locally multiplying cells in various subventricular germinal zones. Bundles of migrating cells enter the parenchyma just posterior to the septal hump and curve toward the front of the brain; however, there are few spindle shaped migrating cells in the fibrous layer that marks the edge of the brain.  These cells may settle in the parenchyma, or turn to migrate perpendicular to the section plane.  Cells are variously oriented in the olfactory bulb parenchyma, but a group of small cells in the dorsal bulb may be migrating AOB output neurons.  The olfactory nerve and the nerve layer in the outermost part of the MOB is full of fibers interspersed with many small cells.  The dashed line between the fibrous layer and the MOB parenchyma indicate the obliterated brain surface.  The outer layer of the olfactory epithelium has cells with a large amount of cytoplasm and many mitotic figures.  This may be where the olfactory sensory neurons are accumulating—the cytoplasm may be spun out to form the long axons these neurons send into the olfactory nerve.  In Jacobson’s organ, bundles of fibers are at the outer edges of the sensory epithelium, and individual fascicles of the vomeronasal nerve grow upward toward the brain.