Developmental pathway of the calretinin(+)-S100(+) subfraction of crypt cells population in the olfactory organ of Poecilia reticulata from birth to sexual maturity

Crypt cells are the most recently described morphotype of fish olfactory neurons, not found in tetrapods. These cells are less abundant than ciliated and microvillar ones in the sensory epithelium. They are suspected to be involved in reproductive behaviours, even if electrophysiological investigations demonstrated some sensitivity to amino acids but failed to identify response to pheromones. In adult Poecilia reticulata the number of crypt cells is different in the two sexes. Using immunohistochemical markers, Gαolf, calretinin and S100 to identify ciliated, microvillar and crypt cells, we compared the size of their populations in juvenile guppies at various steps from birth to 90 days, when their gonads reach maturity. We observed that the number of crypt cells is sex specific, with independent developmental dynamics between males and females, while ciliated and microvillar cell populations have similar pathways. In the guppy calretinin appeared to be expressed also in a subfraction of S100-positive crypt cells (calretinin(+)-S100(-) crypt cells were not observed). We examined the changes in the amount of calretinin(+)-crypt neurons during growth: cell density reflects the variations observed in the whole population of S100(+) crypt cells in both sexes in the first 45 days. However it decreases reaching 3 months of age, in contrast with the increment registered for calretinin(-)-S100(+) cells. We estimated the total number of calretinin(+) crypt neurons and we observed that it remains very close to the value calculated for all crypt cells until 21 days, but it only slightly rises in the next 2 months, while calretinin(-) crypt neurons sensibly increment their number. We hypothesize that this new identified phenotype, percentually more represented in the first weeks of life than later, could be involved in early non-reproductive function. The known sensitivity for amino acids might indicate a role in the perception of food-related stimuli. On the contrary, calretinin(-) crypt cells become predominant reaching sexual maturity, in line with their supposed involvement in reproductive activity.