Degranulation of eosinophilic granular cells with possible involvement in neutrophil migration to site of inflammation in tilapia

Observations were made on stretched preparations from the swim bladder and peritoneum obtained from tilapia (Oreochromis niloticus) after injection of formalin-killed Escherichia coli, proteose peptone, compound 48/80 or HBSS into the swim bladder. The eosinophilic granular cells (EGCs) in the swim bladder degranulated rapidly after inoculation. However, the peritoneal EGCs did not degranulate, indicating that degranulation occurs only at the injected site. There was also a correlation between the ratio of the degranulated EGCs and number of the exudate neutrophils. Killed E. coli (1 mg/fish) produced the greatest degranulation response of EGCs and migration of neutrophils into the inflammatory site. Additionally, the rate of the degranulation and number of the neutrophils were lowest when HBSS was injected. The results of this study verify that degranulation of EGCs is involved in the neutrophil migration and suggest that fish EGCs are analogous to mammalian mast cells. (C) 1999 Elsevier Science Ltd. All rights reserved.