Rainbow trout (Oncorhynchus mykiss Walbaum) intestinal eosinophilic granule cell (EGC) response to Aeromonas salmonicida and Vibrio anguillarum extracellular products

Eosinophilic granule cells (EGCs) are considered to be analogous to mammalian mast cells. In this study the short term ultrastructural responses of ECGs to extracellular products (ECPs) of Aeromonas salmonicida or Vibrio [Listonella] anguillarum were investigated. Bacterial ECP was prepared from a broth culture of bacteria and injected intraperitoneally at a dose of 0.07 and 0.08 micro g/g body weight into 0+ rainbow trout. Controls were injected with saline alone. Fish were killed at 30 min, 1, 2 and 4 h post-injection and the intestine removed, fixed and embedded in resin for light and electron microscopy. Cell counts were made of the number of observable EGCs/mm of the stratum granulosum of the intestine. A rapid decrease in the number of observable EGCs in the gut of ECP-injected fish occurred but there was no apparent change in the number of EGCs in the control fish. This decrease was coincident with an explosive, anaphylactic-like degranulation and a complex multivesiculated substructuring of the EGC granules. In addition there were many ultrastructural changes in the multivesiculated EGCs, including the presence of numerous smooth walled cytoplasmic tubules, spherical electron-lucent vesicles in the peripheral cytoplasm, and a planar subfractionation of the granules which occasionally appeared crystalline. This is the first time that this multivesiculated type of degranulation has been reported in response to A. salmonicida and V. anguillarum extracellular products and is similar to a previously described EGC response to capsaicin and substance P. The significance of this degranulation morphology with respect to the pathogenesis of disease is discussed..