3H-thymidine injections from E15 to E16 and survival to embryonic day (E)21
Figure 25. In the accessory olfactory bulb, there are very few cells heavily labeled in the output layer and in the external plexiform layer. The internal plexiform layer has more labeled cells, most of them small. The small labeled cells may be proliferating glia, and some are probably still migrating radially in the internal plexiform layer. The vomeronasal nerve layer is packed with labeled small cells—probably interneurons and glia that surround the contact zones between the dendrites of the output neurons and the vomeronasal nerve terminals. The dense accumulation of heavily labeled cells in the granular layer that stand out from the main bulb granular layer delineates the granule cells associated with the accessory rather than the main olfactory bulb. In the main olfactory bulb mitral cell layer, there are several large unlabeled cells, many in a superficial position. The internal plexiform layer is full of large labeled cells that are most likely mitral cells migrating into the layer. Unlabeled large cells in the deep half of the external plexiform layer are tufted cells. Most cells (both large and small) are labeled in the superficial half of the external plexiform layer and throughout the olfactory nerve layer and among the fibers in the olfactory nerve.
The neuroepithelium and the granular layer are filled with heavily-to-lightly labeled cells. Just like every other specimen on E21, cells spill into the olfactory recess of the lateral ventricle.