The intracellular localization of heavy metals using autometallography (AUM) was studied in the gills and the digestive gland of the common mussel Mytilus galloprovincialis, after an experimental exposure to 0.1 mg L(-1) of Hg, 0.1 mg L(-1) of Ag, 0.1 mg L(-1) of Pb, and 0.1 mg L(-1) of Cu for 98 days. In the gills, autometallographical black silver deposits (BSDs) were localized in the dense bodies observed in the apical and in the basal part of the cells. Among metals, Hg presented the highest accumulation, followed by Ag, Pb, and Cu. BSDs were more prominent in the abfrontal part of the gill filament in the case of Hg exposure and in the frontal part in the case of Ag and Pb exposure. In the digestive gland, Hg and Ag were localized in the heterolysosomes and the residual bodies of the digestive cells, as well as in the dense bodies of the basophilic cells. The heavy metal exposure also affected the gross morphology of the examined tissues and resulted in the fusion of residual bodies in the digestive cells, the fragmentation or vacuolization of the rough endoplasmic reticulum, and the increase in the number of granules in the basophilic cells. In the gills, fusion of the gill filaments was also noted.
The epithelium of the digestive tubules of the mussel Mytilus galloprovincialis is comprised of two cell types, namely digestive and basophilic cells. In basophilic cells, the secretory granules are beta-glucuronidase immunoreactive, a fact that enhances the hypothesis that beta-glucuronidase is synthesized in basophilic cells. A novel observation at the ultrastructural level is the pinocytic activity associated with the formation of coated pits. This observation constitutes direct evidence for endocytic processes taking place in basophilic cells. The use of cryostat sections from the same digestive tubules reveals, in many instances, a very pronounced neutral lipid accumulation in the same structures giving a positive reaction for N-acetyl-beta-hexosaminidase, indicating the association of those lipids with lysosomes. In some mussels, a high content of lipofuscin was observed in the lysosomes of the digestive cells. In these cases, the lysosomal structures show a limited neutral lipid content, and a weaker N-acetyl-beta-hexosaminidase reaction. In the digestive cells, the carbohydrate content of the lysosomes, and very well-developed canal system in the apical part of cells are discussed in relation to their function.
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