Ultrastructural study of the response of eosinophil granule cells to Aeromonas salmonicida extracellular products and histamine liberators in rainbow trout Salmo gairdneri Richardson.

Intraperitoneal injections of Aeromonas salmonicida extracellular products (ECP), Compound 48/80 and Concanavalin A were found to degranulate the eosinophil granule cells (EGC) in the lower intestine and rectum of the rainbow trout Salmo gairdneri Richardson. Ultrastructurally, the EGC response resembled the anaphylactic granule extrusion of mammalian mast cells. Varying degrees of granule vacuolation and loss of electron density occurred. Labyrinthine channels were observed at the peak of degranulation. EGC response however, differed from mammalian mast cells in two respects. Firstly, degranulation involved the release of intact electron lucent granules and the subsequent disintegration of the granule matrix extracellularly. Mammalian mast cells on the other hand, release their granules by direct exocytosis. Secondly, the 48/80 and Con A-stimulated EGC degranulation was inhibited by antihistamines, promethazine and cimetidine. In mast cells, antihistamines do not prevent granule release but block histamine receptors in target cells. The degranulation of the ECGs was a non-cytotoxic event and the cells were capable of regeneration. As soon as the cells lost most of their granules, increased cytoplasmic activity was observed. This involved the expansion of the Golgi-endoplasmic reticulum complex.