Cellular alterations in collembolan midgut cells as a marker of heavy metal exposure: ultrastructure and intracellular metal distribution

The ultrastructure of the midgut epithelial cells of Tetrodontophora bielanensis (Collembola) fed either with lead-, cadmium- or zinc-enriched food or kept under control conditions was compared by transmission electron microscopy (TEM). Atomic absorption spectrophotometry (AAS) showed accumulation of these three metals in the body tissues of the collembolans. Intracellular localization of zinc in the midgut epithelial cells was investigated by electron spectroscopic imaging (ESI) and electron energy loss spectroscopy (EELS). The presence of lead in the midgut tissue was shown by laser microprobe mass spectrometry (LAMMS). Under heavy metal conditions, the midgut cells showed different ultrastructural alterations, the degree of which was found to be dose-dependent. Independent of the type of the metal in the food, the endoplasmic reticulum appeared fenestrated or vesiculated, and often large vacuoles, which were shown to be ER-derived, occurred throughout the cells. Also the mitochondrial membranes were affected by heavy metal stress. In epithelial cells of individuals exposed to either lead or cadmium, an increase of myelin-like structures could be observed. In the case of exposure to zinc in the highest applied concentration, the cytoplasm showed condensation and portions of the microvillous border appeared destroyed. Additionally, an increase of mineral congregations (type-A spherites) could be observed under heavy metal influence. Intracellularly stored zinc could be localised at highly affected mitochondrial membranes, in the microvillous border, and in the heterochromatin.