Evidence for intrinsic development of olfactory structures in Pax-6 mutantmice

It has been reported that the arrival of primary olfactory axons is require d to induce the development of the olfactory bulb (OB). On the other hand, the Sey(Neu)/Sey(Neu) mutant mouse (Small eye) has been previously describe d as a model for the absence of olfactory bulbs, owing to the lack of olfac tory epithelium (OE). In the present report, we take advantage of this muta nt and study a neural structure in the rostral pole of the telencephalon th at phenotypically resembles the prospective OB. We named this formation olf actory bulb-like structure (OBLS). We also report the occurrence, in the mu tants, of small epithelial vesicles in the malformed craneofacial pits, res embling an atrophic OE, although a mature olfactory nerve was not identifie d. Axonal tracing, birthdating, immunohistochemistry, and in situ hybridiza tion using antibodies and probes expressed in the olfactory system, indicat ed that two distinct structures observed in the OBLS correspond to the main and accessory olfactory bulbs of the control mouse. We propose that the OB LS has developed independently of the external influences exerted by the ol factory nerve. The presence of a prospective OB in the mutants, without int ervening olfactory fibers, suggests that intrinsic factors could define bra in territories even in absence of the proper afferent innervation. The intr insic mechanisms and environmental cues in the telencephalon could be suffi cient to promote axonogenesis in the projection neurons of the OB and guide their axons in a lateral prospective tract, in the absence of olfactory ax one. (C) 2000 Wiley-Liss, Inc.