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Methacrylate-embedded E18 embryo (medial sagittal)

Figure 82.  This sagittal slice of the medial part of the olfactory bulb cuts the anterolateral and posteromedial parts of the basal ganglionic eminence.  Layers in the main and accessory parts of the olfactory bulb are more clearly defined than on E17.  The densely-packed output neurons of the AOB are sandwiched between the external and internal plexiform layers on the dorsal surface.    The thick internal plexiform layer is contiguous with the same layer in the MOB.  In the MOB proper, the outer edge of the NEP columnar cells is easy to distinguish from the variously-oriented cells in the olfactory SVZ.  Note that cells are oozing into the olfactory recess of the lateral ventricle—a common feature of older E18 specimens; this phenomenon is mysterious because we do not know why it happens—but every E19 specimen has it as we will see in the next group of images.  The superficial parts of the internal plexiform layer have radially-oriented cells that will join the growing mitral cell layer.  A fibrous zone—the external plexiform layer—lies in between the outer mitral cells and the olfactory nerve layer.  Migrating cells entering the internal plexiform layer from the basal telencephalon no longer form a prominent stream as on E16 and E17.  Many small bundles of olfactory nerve fibers exit the olfactory epithelium and coalesce into a thick ventral olfactory nerve layer that thins as it continues to the dorsal surface of the bulb.  The epithelium itself is filled with active proliferating cells.  Mitotic figures are present both superficial and deep, matching the label uptake pattern in the 2hr survival autoradiography specimens.