Effects of chronic low-level copper exposure on ultrastructure of the olfactory system in rainbow trout (Oncorhynchus mykiss).

This study investigated the effects of a chronic exposure to a low level of copper on cell populations of the olfactory system in yearling rainbow trout. Fish were sacrificed after 15, 30 and 60 days of copper exposure. Transmission electron microscopy was used to describe the sequence of subcellular changes occurring in three tissues, the sensory epithelium, the olfactory nerve and the olfactory bulb. Data show that a 15-day exposure to 20 micrograms/l of copper causes specific degeneration of all mature receptor cells as well as numerous immature neurons. Moreover, degenerating receptor cells exhibited morphological features of a cell death by apoptosis. After 30 days, and more specifically after 60 days of exposure, numerous clusters of cells were observed in the basal region of the epithelium, suggesting a great mitotic activity in this area. In parallel, an increased number of maturing receptor cells and goblet cells were observed, but no fully mature neurons were noted even after 60 days of exposure. In both the olfactory nerve and the olfactory bulb, the number of degenerating axons and terminals, which was high at 15 days, decreased with time and some process of glomerular reinnervation was detected after 60 days. A reactive hypertrophy of supporting, ensheathing and astrocytic cells was also observed in exposed fish, which demonstrates that these cell types are actively involved in the process of tissue scarring. Even though some signs of neuronal regeneration were reported during the time-course of exposure, indicating some fish acclimation, results raise the question of the olfactory function during such environmental stress.