Zinc granulocytes in the oyster, Ostrea angasi

Man's activities impose an increasingly heavy burden of metals on the marine environment. An excess of the transitional element zinc, which is essential for life, is suspected of causing ecological disruption. Before stable zinc can be regarded as a pollutant, harm to marine organisms will have to be demonstrated. Histochemical and physical methods of analysis have been applied to the tissues of the native oyster, Ostrea angasi to determine whether or not the accumulation of zinc is associated with any cytopathological effect or histological change. Oysters were gathered from the south and east coasts of Tasmania and compared with specimens collected from sites in the Derwent Estuary, where commercially grown oysters containing up to 10% of their dry weight as zinc, had previously been responsible for cases of food poisoning. Tissue was prepared for light and.electron microscopy by fixation in glutaraldehyde saturated with hydrogen sulphide. The concentration of zinc in the visceral mass was determined by atomic absorption spectrophotometry and zinc in adjacent tissue located histochemically by the sulphide-silver and alkaline-dithizone techniques. Resin-embedded thin sections of the intestinal tract were examined in a transmission electron microscope and both thin and thick sections were analysed in a combined scanning electron microscope-electron probe microanalyser. Zinc was located intracellularly as electron dense granules 1 gm in diameter. In the scanning electron microscope these granules were electron reflective and appeared as bright spherules, which in the transmission electron microscope had a ring-like structure. The presence of zinc in these granules was confirmed by energy dispersive and wavelength dispersive systems of X-ray analysis. Zinc-laden granulocytes were widely distributed throughout the connective tissue and vascular spaces and concentrated beneath the epithelia of the intestinal tract, palps, digestive diverticula, gills and mantle. The granulocytes were also conspicuous within these epithelia. There was no evidence of a pathological tissue reaction, although in some instances large collections of granulocytes beneath the intestinal epithelium were associated with abundant fibrous connective tissue. The concentration of zinc in the visceral mass was - the same order of magnitude ( 10 4pg g dry weight ' ) in all specimens. Circumstantial evidence suggests that each specimen came from a 'high-zinc' environment, despite their wide geographical separation. Specimens would have to be obtained from a proven /low-zinc' environment before definite conclusions can be drawn from the observed histological features. The function of the zinc granulocyte is in debate. One school of thought claims that the cell is part of a microscope-electron probe microanalyser. Zinc was located intracellularly as electron dense granules 1 gm in diameter. In the scanning electron microscope these granules were electron reflective and appeared as bright spherules, which in the transmission electron microscope had a ring-like structure. The presence of zinc in these granules was confirmed by energy dispersive and wavelength dispersive systems of X-ray analysis. Zinc-laden granulocytes were widely distributed throughout the connective tissue and vascular spaces and concentrated beneath the epithelia of the intestinal tract, palps, digestive diverticula, gills and mantle. The granulocytes were also conspicuous within these epithelia. There was no evidence of a pathological tissue reaction, although in some instances large collections of granulocytes beneath the intestinal epithelium were associated with abundant fibrous connective tissue. The concentration of zinc in the visceral mass was - the same order of magnitude ( 10 4pg g dry weight ' ) in all specimens. Circumstantial evidence suggests that each specimen came from a 'high-zinc' environment, despite their wide geographical separation. Specimens would have to be obtained from a proven /low-zinc' environment before definite conclusions can be drawn from the observed histological features. The function of the zinc granulocyte is in debate. One school of thought claims that the cell is part of a metal detoxification system, whereas others believe that the cell plays a physiological role in the cellular defence mechanism of the host against injury. The precise function of these cells will have to be established before definitive statements can be made about contamination of oyster tissue by zinc.