Ultrastructural and histochemical aspects of zinc accumulation by fish scales

The effect of zinc exposure on the ultrastructure of the scales and scale associated cells of the estuarine teleost, Fundulus heteroclitus was investigated in laboratory experiments. The Timm sulfide silver stain indicated that in the calcified region of the scales, Zn was colocalized with the calcium phosphate mineral crystals. X-ray diffraction analysis showed that Zn did not have an effect on crystal structure. The scale osteoblasts of Zn-exposed fish showed an increase in the number of lysosome-like structures contained by the cytoplasm. In Zn-exposed animals, X-ray microanalysis revealed that these structures contain greatly increased levels of zinc and sulfur relative to controls. In all specimens, the lysosomes contained higher levels of Zn than either the surrounding cytoplasm or adjacent scales. The findings suggest that osteoblast lysosomes may be involved in the accumulation of Zn and other metals by fish scales by the enzymatic degradation of metallothioneins or other metal-binding proteins. This could represent an important mechanism for the detoxification of excess heavy metal ions taken up from the environment and the metabolism of essential metals by calcified tissues.