STUDIES ON THE EOSINOPHILIC GRANULE CELLS IN THE GILLS OF THE RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

Eosinophilic granule cells (EGCs) found in the gills, skin and aliment ary canals of fish have been likened to mammalian mast cells in terms of their structure and function. To investigate this situation further , gill explant cultures from the rainbow trout, Oncorhynchus mykiss, w ere set-up and incubated with either lipopolysaccharide (LPS; 5 mu g m l(-1)) or human recombinant tumour necrosis factor-alpha (TNF-alpha; 2 5 iu ml(-1)) alone or in combination for 7 days. Examination of histol ogical sections of these gill explants after this incubation showed a significant increase in the number of EGCs in those explants incubated with a combination of LPS and TNF-alpha compared with the control. Si milarly, exposure of trout to short-term (> 6 h) handling and confinem ent stress resulted in a significant increase in the number of EGCs in the gills, while longer term stress (> 6 days) was without significan t effect. The EGCs in the gills were shown to contain granules that re acted with both basic dyes, such as methylene blue, and eosin but fail ed to react with periodic acid Schiffs reagent. Of particular interest was the finding that only some of the EGCs reacted with the leucocyte -specific monoclonal antibody, 21G6, suggesting some heterogeneity wit hin this cell type in the gill. (C) 1998 Elsevier Science Inc. All rig hts reserved.