THE LEUKOCYTE POPULATION OF THE UNSTIMULATED PERITONEAL-CAVITY OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

The leucocyte population of the resting (unstimulated) peritoneal cavi ty of rainbow trout (Oncorhynchus mykiss) weighing from 100 to 200 g a nd kept at 10, 15 or 20 degrees C was analysed. The values for total l eucocytes were not significantly different for the three temperatures studied, the average being 4.35 +/- 1.76 x 10(6) per cavity. In perfor ming differential counts of Wright-stained preparations, 9 to 13% of t he cells could not be characterised with confidence (because large lym phocytes and small macrophages are impossible to distinguish), 33 to 3 9% were macrophages, 46 to 53% were small lymphocytes and 1.3 to 2.2% were neutrophil granulocytes. Electron microscopy observations suggest that most of the cells which were difficult to characterise by light microscopy would be small macrophages. Thrombocytes (identified by ele ctron microscopy) and eosinophilic granular cells (EC;C) were very rar ely seen. Light microscopy cytochemistry showed that: (1) macrophages and some lymphocytes were non-specific esterase positive; (2) macropha ges and a few lymphocytes were acid phosphatase positive; (3) macropha ges and lymphocytes were alkaline phosphatase negative and peroxidase negative; and (4) neutrophils were alkaline phosphatase and peroxidase positive. This latter characteristic allows for the clear distinction between macrophages and neutrophils, the strong, granular peroxidase staining being particularly evident. Transmission electron microscopy of ultrathin sections of samples processed for peroxidase activity sho wed that the neutrophil cytoplasmic granules, irrespective of size and shape, were positive, while all the other leucocytes were negative. T he characteristic peroxidase staining of the nuclear and rough endopla smic cisternae found in mammalian resident peritoneal macrophages was not seen in the resident macrophages of the resting rainbow trout peri toneal cavity. Ultrastructural cytochemistry for acid phosphatase show ed positively stained cytoplasmic granules in the macrophages, in neut rophils and in a few small lymphocytes. (C) 1997 Academic Press Limite d.