Calretinin immunoreactivity in the developing olfactory system of the rainbow trout

The distribution of calretinin immunoreactivity in the developing olfactory system of the rainbow trout was studied by using an indirect immunocytochemical method. Calretinin immunoreactivity was firstly detected at 150 day-degrees in the olfactory placode, where labeled primordial cells were observed. At 250 day-degrees, precursor cells of the olfactory receptor neurons located in the olfactory pit were calretinin-immunoreactive. At 300 day-degrees, recognizable olfactory receptor neurons displayed calretinin immunoreactivity in the olfactory epithelium, and calretinin-immunopositive olfactory axons reached the presumptive olfactory bulb. After hatching (400 day-degrees) and during the subsequent development and maturation of the olfactory system, the number of calretinin-immunopositive olfactory receptor cells increased and distributed homogeneously throughout the olfactory epithelium. Accordingly, new positive olfactory fibers arrived to the olfactory bulb arborizing in olfactory glomeruli distributed in nine different terminal fields. Six days after hatching, calretinin-immunopositive interneurons within the olfactory bulb were also observed. The size and number of calretinin-immunoreactive interneurons increased from this stage to adulthood. The adult pattern demonstrated both similarities and differences with the distribution of calretinin immunoreactivity previously described in the olfactory system of mammals.