The leucocyte population of the unstimulated peritoneal cavity of rainbow trout (Oncorhynchus mykiss)

The leucocyte population of the resting (unstimulated) peritoneal cavity of rainbow trout (Oncorhynchus mykiss) weighing from 100 to 200 g and kept at 10, 15 or 20° C was analysed. The values for total leucocytes were not significantly different for the three temperatures studied, the average being 4·35±1·76×106per cavity. In performing differential counts of Wright-stained preparations, 9 to 13% of the cells could not be characterised with confidence (because large lymphocytes and small macrophages are impossible to distinguish), 33 to 39% 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 electron microscopy) and eosinophilic granular cells (EGC) were very rarely seen. Light microscopy cytochemistry showed that: (1) macrophages and some lymphocytes were non-specific esterase positive; (2) macrophages and a few lymphocytes were acid phosphatase positive; (3) macrophages 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 showed that the neutrophil cytoplasmic granules, irrespective of size and shape, were positive, while all the other leucocytes were negative. The characteristic peroxidase staining of the nuclear and rough endoplasmic cisternae found in mammalian resident peritoneal macrophages was not seen in the resident macrophages of the resting rainbow trout peritoneal cavity. Ultrastructural cytochemistry for acid phosphatase showed positively stained cytoplasmic granules in the macrophages, in neutrophils and in a few small lymphocytes.