Central olfactory structures in Pax-6 mutant mice.

During the development of the olfactory system, cells located in the olfactory placode/olfactory pit send their axons toward the rostral part of the telencephalic vesicles (TVs). Some of these enter the TV inducing the formation of the olfactory bulbs (OBs), whereas, mitral and tufted cell axons form the lateral olfactory tract (LOT). Our recent studies have shown that the beginning of the central olfactory projections is independent of the arrival of olfactory receptor neuron (ORN) axons to the TV. Here we have used the mouse carrying a mutation in the Pax-6 gene to study whether the nasal olfactory structures intervene in the formation of central olfactory structures. This mutant as well as lacking a nose and eyes, is reported to lack olfactory epithelium and OB. However, we have found an ovoid cellular structure localized in the rostral part of the brain, and some cells in this structure project axons toward the piriform cortex forming a presumptive LOT. We conclude that the referred structure is an OB, which fails to develop because the mutation in the Pax-6 gene affects the formation of nasal structures. As such, fibers of the ORNs are necessary for the protrusion and layered formation of the OB, but these inputs are not necessary for the establishment of the central olfactory projections.