Differential development of the three populations of olfactory neurons in the sensory organ of Poecilia reticulata from birth to sexual maturity.

The olfactory epithelium of fish consists of three types of sensory neurons: the ciliated cells and the microvillar cells (common also to other Vertebrates), and the crypt cells, generally less numerous than the others. In addition to peculiarities in number, distribution and morphology, they are sensitive to different odorants and project to exclusive regions of the bulbar glomerular layer. Ciliated neurons seem to mediate alarm reactions, microvillar cells are involved in feeding behaviour, while crypt cells are supposed to respond to sexual stimuli. However, their roles remain unclear, in particular for crypt cells. In this study we examined if, in the livebearer guppy, the olfactory cell types differ also during development. We focused on the variations in the amount of neurons during the first three months after birth, when males and females reach sexual maturity. Each cell type was immunohistochemically detected: ciliated cells with anti-Gαolf, microvillar cells with anti-calretinin and crypt cells with anti-S100. Crypt neurons were easily counted because of their low number, while densitometric analysis was applied to quantify the amount of Gαolf- and calretinin-positive cells. In females ciliated and microvillar cell populations had the same pathway of development, with a positive peak around 45 days after birth (corresponding with lamellar size growth) before resuming the definitive amount. Males showed a similar, even if statistically more constant, trend. On the contrary, the dynamic developments of the crypt cell population in males and females appeared diametrically opposed: while in males crypt cell density increased up to 3 weeks after birth, in females it decreased, but this situation reversed as their density was higher in females at 90 days. These findings suggest that only crypt cell number is sex specific, a result consistent with the hypothesis that they mediate sexual communication.