Methacrylate-embedded E22 embryo in the coronal plane (anterior)
Figure 141. This anterior coronal slice cuts through the main olfactory bulb of a normal E22 embryo. The entire slice looks much like the slices of the P5 autoradiographic specimens. However, the giveaway that this is an earlier age group is the small cluster of cells shed into the lumen of the olfactory recess. That phenomenon is completely gone by P5. Cells are sorted in concentric layers around the lumen of the olfactory recess of the lateral ventricle: 1) the olfactory NEP/ependyma is the dense innermost layer, 2) the less dense SVZ is difficult to distinguish from the 4) forming granule cell layer, 5) spindle-shaped and radial-oriented cells fill the internal plexiform layer, 6) mitral cells with dense cytoplasm are arrayed in a continuous layer, 7) outside of that is the fibrous external plexiform layer with large tufted cells, 8) finally the olfactory nerve layer is full of cells and circular fiber whorls but no characteristic cell-sparse circular areas that define glomeruli. Glomeruli are features that develop between E22 and P5.