Combined X-Ray analysis and electron microscopy of zinc in oyster tissue

Abstract

Contamination of the marine environment with zinc, as well as posing a hazard to human health, is suspected of causing ecological disruption. Histochemical and physical methods of analysis were applied to the native oyster, <i>Ostrea angasi</i>, to determine the cytopathological effects of zinc accumulation within its tissue and to explain its tolerance for this heavy metal. Specimens were 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. Oysters were also gathered from the east coast of Tasmania. Tissue was fixed in H₂S-saturated glutaraldehyde and prepared for light and electron microscopy (CTEM). The concentration of zinc in the peri-intestinal tissue was determined by atomic absorption spectrophotometry and the element located histochemically by the sulphide-silver and alkaline-dithizone techniques. Thin sections were examined in an Hitachi HS-7S transmission electron microscope, and both thin and thick sections were examined in a JEOL JXA-50A combined scanning electron microscope (SEM) and electron probe microanalyser. The concentration of zinc in the intestinal tissue was the same order of magnitude (10⁴<i>μ</i>g/g dry weight) in all specimens and the tissue reaction was the same in all instances. Zinc was widely distributed beneath the epithelium of the digestive tract and located within the cytoplasm of phagocytic cells. These zinc-laden cells were also seen within the intestinal epithelium. Large numbers of phagocytes beneath the basement membrane were associated with a fibrous tissue reaction. Intracellular zinc in the CTEM was in the form of electron dense, ring-like structures (1<i>μ</i>m in diameter), which appeared as bright spherules in the SEM. The presence of zinc in these granules was confirmed by energy dispersive (EDAX) and wavelength dispersive systems of X-Ray analysis.