Acute and subchronic exposure of diplopods to substrate containing sewage mud: Tissular responses of the midgut

Abstract

Several discussions regarding the agricultural use of sewage mud have occurred; however, its use has been questioned due to the risks of soil contamination mainly by metals. Diplopods are saprophagous invertebrates, agile in colonizing several layers of soil. Due to the habit of this group, several researchers have proposed their use in ecotoxicological analyses. This study aimed to expose these invertebrates to substrate containing sewage mud of a STS (Sewage Treatment Station) from São Paulo State, Brazil and analyzed its toxic potential by morphological analysis of the midgut of the diplopod Rhinocricus padbergi, region of the digestive tube where digestion and nutrient absorption occur. The animals were exposed in mud concentrations at 1%, 10% and 50% mixed with soil from the collection site for periods of 7 (acute exposure), 15 (intermediary exposure) and 90 (subchronic exposure) days in each treatment. The animals from the control group did not present alterations at all the exposition times. As an acute response (7 days), it was observed in the animals exposed to mud at 1% and 10% an increase in the epithelial renovation and in the liberation of secretory vesicles of glycoprotein content. In the animals exposed to mud at 50%, the main alteration observed was the increase in the number of cytoplasmatic granules in the fat body cells; the epithelium seemed to be in reorganization process in all the observed individuals, suggesting an intense epithelial renovation. In the intermediary period of exposure (15 days), in the animals exposed to mud at 1% it was observed an increase in the number of haemocytes among the cells of the fat body, distributed both separately and grouped; the animals exposed to mud at 10% presented the same evidence of epithelial renovation observed in the animals exposed for 7 days at a concentration of 50% of mud; it was also observed an increase in the number of haemocytes, which reacted strongly to the PAS technique. Cytoplasmatic granules in the hepatic cells, fat body cells and haemocytes were also observed in great number, being that great part presented calcium in their constitution. In the animals exposed to mud at 50%, the alteration observed was the presence of numerous secretory vesicles of the apocrine type through all the epithelia of vacuolated aspect; hepatic cells with cytoplasmatic granules were also observed. As subchronic response (90 days), it was observed that all the fat body cells were completely taken by cytoplasmatic granules, into which it was, detected the presence of calcium; such granules were also observed in the apical region of the principal epithelial cells. These results suggest the existence of toxic agents in the analyzed mud sample, whose different concentrations trigger responses that occur in different manners, intensities and velocities that might prevent the entrance and action of toxic agents in the organism of the studied invertebrate. (C) 2009 Elsevier Ltd. All rights reserved.