Sequential development and morphology of experimentally induced hepatic melano‐macrophage centres in Rivulus marmoratus

Abstract

Formation of hepatic melano‐macrophage centres (MMCs) in Rivulus marmoratus was induced with the coccidian parasite Calyptospora funduli. Experimental infections were produced by feeding infected intestine and hepatopancreas from grass shrimp (Palaemonetes pugio) to individual parasite‐free fish. Livers of fish sampled 5–150 days post‐infection were examined histologically. Mild diffuse inflammation initiated during early merogony (5–8 days) consisted of eosinophilic granulocytes and heterophils. Liberation of merozoites (8–12 days) caused extensive hepatocyte degeneration and augmented leucocyte exudation. During gamogony (15–18 days) mononuclear phagocytes became a predominant component of the cellular exudate. Focal lesions, considered to be early MMCs, became apparent during early sporogony (20–25 days); they consisted of degenerating infected hepatocytes and inflammatory cells, and contained developing oocysts and degenerating macrogamonts, but no pigment. Early centres became visible grossly at 30 days as diffuse, yellow‐tan foci within the hepatic parenchyma. Pigment content increased progressively, but melanin was never a major component. Oocysts first elicited a granulomatous response between 40 and 50 days. The study suggests that MMC formation in this instance is an inflammatory process in which mononuclear phagocytes recruited from peripheral circulation play a dominant role. MMC development appears to be elicited by macrogamont degeneration, whereas granuloma formation is probably a response to oocysts.