Methacrylate-embedded E15 embryo (dorsal horizontal)
Figure 36, a dorsal horizontal slice shows how the presumptive olfactory NEP is sandwiched between the lateral ganglionic and medial septal NEPs. The thickened NEP medially adjacent to the olfactory NEP is loaded with radially migrating cells that approach the pia, in the region of the vomeronasal and olfactory nerve contact zone—probably mostly vomeronasal fibers—then migrate forward (dashed line arrows). That part of the forebrain may be the source of the AOB output neurons that should be migrating in massive numbers to the future AOB. The sagittal sections show that vomeronasal nerve fibers approach the olfactory bulb in a medial position; It makes sense that the AOB migrating neurons are attracted to the very nerve fibers that will contact them in the mature brain. But on E15, the AOB cannot be distinguished from the MOB. The lateral part of the slice is also interesting. Streams of cells are exiting both the lateral and medial parts of the basal ganglionic NEPs. Some cells accumulate in a dense clump beneath the pia; many of these may be the older neurons in layer III of the primary olfactory cortex. Other cells reach the pia and become spindle shaped—indicating migration within the section plane. These spindle shaped cells are migrating in a nascent fiber tract, the lateral olfactory tract (LOT), that is made up of mitral cell axons. It has been shown that mitral cells leave their axons behind as they migrate forward. Of course, one of their main targets are cells in the primary olfactory cortex. So on E15, we already have evidence that major brain circuits are being connected in the basal forebrain.