Methacrylate-embedded E17 embryo (medial sagittal)

Figure 65.  This sagittal slice of the medial part of the olfactory bulb cuts the septum and medial part of the basal ganglionic eminence.  The superior preservation of histological features in methacrylate allow us to see the beginnings of layers in the olfactory bulb.  On the dorsal surface, a dense group of cells can be identified as the output neurons of the AOB.  The cell-sparse layer between the output neurons and the vomeronasal nerve fibers is the external plexiform layer.  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.  There are many spindle-shaped cells in the internal plexiform layer (probably mitral cells) that are apparently migrating around the forward tip of the olfactory bulb; the spindle-shaped cells are more densely packed along the outer perimeter with lots of darkly-stained cytoplasm; that is outlined with dashed lines and is the primitive mitral cell layer.  A fibrous zone—the external plexiform layer—lies in between the outer mitral cells and the olfactory nerve layer.  In this medial slice, we do not see bundles of migrating cells going into the bulb from the basal forebrain.  There are a few small bundles of olfactory nerve fibers outside the olfactory epithelium.  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.