Paraquat embryotoxicity in the Xenopus laevis cleavage phase

The high Paraquat (PQ, 1-1'-dimethyl-4,4',bipyridylium dichloride) embryoto xicity in Xenopus laevis has been shown to be due to its rapid reduction an d instantaneous re-oxidation which produces a reactive oxygen species, ROS. Nevertheless, PQ did not show any effects before hatching, stage 32, which showed a resistance, in early X. laevis development, to oxidative damage. Moreover, in view of its genotoxic properties in several experimental model s, we studied PQ in the X. laevis cleavage phase that, characterized by a s eries of rapid mitotic divisions, might be damaged by genotoxic compounds. Embryos were exposed to 20, 40, 60, and 80 mg/l PQ concentrations from stag e 2 to stage 9, and then left to develop in control FETAX solution until st age 47. The 80 mg/l PQ concentration gave 19% embryo mortality at the end o f the exposure time, and 16.7% larvae mortality at the end of the test; bot h values were statistically different from the control, 5 and 6.8% respecti vely. These results confirmed the high resistance in early X. laevis develo pment to PQ oxidative damage. The malformed larva percentages in the PQ exp osed groups were higher as regards the control value but did not show any c oncentration-response; the most frequent malformed larvae found were affect ed by abnormal tail flexure coupled with abnormal gut coiling. A further ex periment was carried out using the same methodology, but exposing embryos o nly to the 80 mg/l PQ concentration. The surviving blastulae were embedded in Paraplast, then the slides were stained with 4',6-diarnidino-2-phenylind ole (DA-PI) and the nuclei were examined with a confocal microscope. This n ew preliminary procedure did not reveal any significant presence of micronu cleated micromeres in PQ exposed blastulae with respect to the control. Nev ertheless, the mechanism by which PQ induced abnormal tail flexure after cl eavage exposure remained unknown. PQ seemed to pass through the jelly coats and vitelline membrane, but it expressed teratogenicity between the 2nd an d 3rd day. PQ might be accumulated in the embryos during the exposure, and might express teratogenicity later, but it did not seem to induce genotoxic ity during the cleavage phase of X. laevis even at very high concentrations . (C) 2001 Elsevier Science B.V. All rights reserved.