Methacrylate-embedded E16 embryo (anterior coronal)
Figure 55. This slice is cut in the coronal plane of a normal embryo killed on the morning of E16. The left side of the slice cuts through the main olfactory bulb (MOB), while the right side cuts farther back where the olfactory bulb merges with what is probably the cerebral cortex; both sides are labeled. Regarding the labels: there is little laminar definition in the olfactory bulb so all identifications of cells are “educated guesses.” The fibrous and cell-dense olfactory nerve layer, which is the most certain label, surrounds most of the left MOB. Beneath the nerve layer is a less-dense cellular zone with copious fibers—the best identification we can make here is that it is the external plexiform layer, although it probably contains fibers that will become part of the glomerular layer where mitral cell dendrites contact the afferent axons of the olfactory nerve. The core of the left MOB is full of large spindle-shaped cells, mostly horizontally oriented mitral cells migrating within the plane of the section. These are the same cells that cap the bulb terminus that we saw in the sagittal and horizontal slices. The right part of the slice shows the same layers ventrally in the presumptive MOB. A middle part (set off by dashed lines) is very tentatively identified as the AOB output neurons, and above that is a tangential cut of the primordial plexiform layer in the cerebral cortex. The olfactory epithelium shows the same features as in the sagittal slices (Figs. 49-50): small bundles of the olfactory nerve outside the epithelium itself, a layer of cells with large amounts of cytoplasm interspersed with actively multiplying cells, and a columnar layer of cells that looks very much like the neuroepithelium in the central nervous system. Note the change in cellular orientation, layer thickness, in what is probably the nasal mucosa lining the central parts of the nasal cavity. The hook of thickened epithelium in the ventromedial cavity is probably the most anterior part of Jacobson’s organ.