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Methacrylate-embedded E14 embryo (ventral horizontal)

The slice in Figure 22 grazes the most ventral part of the forebrain.  It is my best guess that we are below the septal NEP and the presumptive olfactory NEP lies adjacent to the midline and is contiguous with the bulging basal telencephalic NEP.  There is a prominent accumulation of spindle-shaped cells (outlined) near the surface of the brain in the basal telencephalon.  Only around 10% of the mitral cells have been generated by the morning of E14, and this large clump of apparently migrating cells may be another group of early generated neurons in the basal telencephalon.  The forebrain has a very immature structure on E14.  We know from the autoradiographic studies that the output neurons in the accessory olfactory bulb is the only population with substantial neurogenesis before E14.  Perhaps these spindle-shaped cells are AOB output neurons?  But at later ages, we will see more and more migrating cells in this same location that are definite mitral cells.  Thus, the identity of this clump of cells will have to depend on future discoveries of cell-specific markers to distinguish AOB output neurons, MOB mitral and tufted cells, and other basal telencephalic neurons.  Note that the olfactory epithelium is continuous with the site of invagination that is narrowing into a future nostril.  The anteromedial part of the epithelium, with its hook-like curve, is Jacobson’s organ—the source of the vomeronasal nerve; the vomeronasal nerve exiting Jacobson’s organ is visible in slices below the brain.  The posteromedial epithelium is associated with the olfactory nerve.  The lateral epithelium will form the nasal mucosa.