Immunohistochemical characterization of the crypt neurons in the olfactory epithelium of adult zebrafish

The fish sensory epithelium contains three types of sensory cells denominated ciliated, microvillous, and crypt neurons. Each one differs from the other in its morphological, ultrastructural and molecular features, as well as in their projections to the central nervous system. Crypt neurons are present in both bony and cartilaginous fish and can be identified on the basis of their morphology and the expression of some specific proteins and genes. In this study we have investigated the morphology of crypt neurons, as well as the occurrence and co-localization of S100 protein, calretinin and TRPV4, three proposed markers for crypt cells, in the olfactory epithelium of adult zebrafish (Danio rerio) using double immunofluorescence associated to laser confocal microscopy. A sparse population of superficial S100 protein positive cells was detected being identified as crypt neurons. The calretinin immunoreactive cells were more abundant, occasionally resembling the morphology of the crypt cells but never displaying co-localization of both proteins. The TRPV4 positive cells differed in morphology from crypt cells, thus excluding the occurrence of TRPV4 in those cells. These results demonstrate that only S100 protein immunoreactivity can be used to identify crypt cells. Because some calretinin positive cells showed localization and morphology similar to the crypt cells of the sensory epithelium, the occurrence of two subtypes of crypt cells, S100 protein and calretinin positive, cannot be excluded. The significance of these findings remains to be elucidated.