Expression of bHLH Transcription Factors and IGFs in the Non-sensory Patches, Olfactory Epithelium and Vomeronasal Organ

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Non-sensory patches in the posterior dorsal fossa of the rodent nasal cavity are formed between postnatal (P) days 10 and 21 due to programmed death of ORNs and GBCs (Suzuki et al., 2000). In a previous study, we found bHLH transcription factors Hes6 and NeuroD to be expressed in the GBC layer in the olfactory epithelium (OE) of mice. During embryonic days, there were no regional differences in the expression of these two genes, whereas at postnatal days 3–7, expression disappeared in the region corresponding to the presumptive non-sensory patches (Suzuki et al., 2003). Hes 6 and NeuroD are known to promote neuronal differentiation, NeuroD acting downstream of Mash1 and Neurogenin 1, which are determination genes for ORNs. Mash 1 null mutant mice failed to produce progenitor cells and Neurogenin 1 null mutant mice, progenitors of ORNs are generated but their differentiation is blocked (Cau et al., 2002). It is not yet known whether Mash1 and Neurogenin1 are related to the formation of non-sensory patches. On the other hand, insulin-like growth factor (IGF) family and related molecules were reported to be expressed in the rat OE where IGFs may have functions of differentiation and survival of ORNs (Suzuki and Takeda, 2002). Six high-affinity IGF binding proteins (IGFBP-1 to -6) modulate IGF effects. In the present study, we examined whether the expression of bHLH transcription factors and IGFBPs changes in the region where the programmed death of ORNs and GBCs occurs.