The olfactory bulbs were not different across groups apart from the much smaller olfactory bulbs in females under unrestricted feeding.
Thus, at E11.5 loss of N-myc reduces Notch activity, proliferation, and ongoing neurogenesis, resulting in a smaller olfactory epithelium.
In addition, stereological analyses revealed reduced cell size of post-mitotic neurons in the olfactory epithelium, which contributed to a smaller olfactory pit.
First changes in N-myc mutants were detected at E11.5, with reduced proliferation and neurogenesis in a slightly smaller olfactory epithelium.
To further examine the smaller olfactory pit and vomeronasal organ in N-myc-deficient mice, we analyzed the size of single cells in these structures.
Furthermore, the large size of the Latimeria T2R repertoire, comparable to some of the smaller olfactory receptor gene families, cf. [ 8], suggests that the sense of taste may require unexpectedly high molecular complexity.
Important to note is that apoptosis was not the cause for the smaller olfactory epithelium, since the number of Caspase3+ cells was similar in N-myc −/− compared to wild-type mice (Supporting Information Fig. S3A).
Our data at E13.5 indicate a progressive effect of N-myc deficiency, in which decreased proliferation, reduced Notch activity and depletion of the Hes5+ progenitor cells results in a significantly smaller olfactory pit and suppressed neurogenesis.
A similar explanation for the smaller olfactory gene number in great apes relative to rodents has been proposed, based on an increased emphasis on a sophisticated visual system [ 20].
Further work could also address whether nocturnal birds that invested in enhanced visual or auditory perception have smaller olfactory receptor repertoires, which would suggest a trade-off between investments in vision versus olfaction.
