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E13 methacrylate-embedded embryo (medial sagittal slice)

Figure 8 is a medial sagittal slice that tangentially cuts the invaginating olfactory placode.  The mitotic cells in the superficial part of the placode are in the presumptive olfactory epithelium.  Just outside the epithelium is a dense group of mesenchymal cells.  These cells may be generated in the olfactory epithelium itself and migrate outwards.  The cells have dark-stained cytoplasm, and the clumps of non-nucleated rounded material may be cytoplasm outside the nucleus.  There is a continuum between the epithelium and the dense cells.  Could these cytoplasmic extensions from the epithelium be the pioneer fibers of the olfactory nerve?  The mesenchymal cells in the clump may be waiting there to bundle the fibers into fascicles of the olfactory nerve as it grows toward the basal telencephalon.  In the telencephalon, there is no discernible olfactory bulb, and it is hard to see the superficial border of the NEP.  The dashed line shows the best guess where the NEP ends and the zone of accumulating postmitotic neurons begins.  Note that the posterior parts of the basal telencephalon have a thicker zone of accumulating cells—from the autoradiography we know these are neurons generated on E12.  The few cells outside the presumptive olfactory NEP may be AOB output neurons.