Seasonal changes in intra- and interorgan occurrence of rodlet cells in freshwater bream

Significant seasonal differences in intra- and interorgan prevalence, abund ance and distribution of rodlet cells (RC) were observed in freshwater brea m caught in May and August 1997 from a PCB-polluted lake. Lake Kernaala, Fi nland. In May, the majority of RCs in liver were seen in blood vessel endot helium, while in August most cells were located in the epithelium of the in trahepatic biliary ducts. In kidney, a significant increase in RCs in the c olumnar epithelium of the collecting ducts was observed from May to August. RCs in spleen were extremely rare in May but were seen frequently in the e ndothelium of the splenic blood vessels in August. In gill and intestine, R Cs were found in every tissue sample studied, with no statistical differenc es in their abundance or distribution between seasons. No RCs were seen in gonads. Total numbers of RCs increased dramatically from May to August in k idney and in spleen. The frequency of RCs corresponded strongly with the cu rrent phase of parasitic infections in liver, kidney, and spleen. It is con cluded that RCs play a rule in host defence mechanisms in fish, displaying induced expression at the times of peak parasitic prevalences. (C) 2000 The Fisheries Society of the British Isles.