Methacrylate-embedded E14 embryo (intermediate horizontal)
Figure 21 is an intermediate level horizontal slice through the basal forebrain. The nasal cavity is larger because the slice cuts a deeper part of the olfactory epithelium. Note that the posteromedial epithelium is the only part that is associated with the dense cells and sparse fibers of the olfactory nerve. As in Figure 20, the presumed vomeronasal nerve is in three discrete anteromedial bundles. In the basal forebrain, the NEP is sliced tangentially, exaggerating its thickness. The labels for different parts of the NEP are our best estimate as to the location of germinal sources for the septum medially and the laterally adjacent olfactory NEP that blends into the NEPs of the basal ganglionic eminence. This slice shows a large accumulation of neurons generated on E12 and E13. The spindle-shaped cells near the surface of the brain (outlined) may be pioneer mitral cells migrating toward the part of the brain that will grow forward to form an olfactory bulb. It is also possible that these cells are destined to settle in other parts of the basal forebrain—layer III of the primary olfactory cortex, various magnocellular basal telencephalic nuclei, and ventral parts of the globus pallidus. Hopefully, future discoveries of cell-specific markers for various neuronal groups in the basal forebrain will identify the heterogeneous mass of neurons outside the eminence on E14.