| Perfluorooctane sulfonate(PFOS) has been widely used as surfactants in various industry and consumer products,because of its unique properties as repellents of dirt,water and oils.PFOS is very persistent in the environment and has bioaccumulation effect.As one of emerging chemicals, PFOS has been discovered as global pollutants in various environmental mediums and biology. The acute toxicity of PFOS is moderate,it has different toxicity for various internal organs,and may be an emerging environmental endocrine disrupting chemincal.Xenopus laevis,as the most typical model animal of vertebrate developmental biology,has also tremendous advantage in testing endocrine disrupting effects.Xenopus laevis has been gradually gotten recognition in studying environmental endocrine disrupting effects as a model animal.In present paper,firstly we used Amphibian Metamorphosis Assay(AMA) to reveal thyroid hormone disrupting effects of PFOS according to the criterion of OECD.X.laevis tadpoles were exposed to PFOS at the concentrations of 0.01,0.05 and 0.25 mg·L-1 from NF51 to NF58 for 19 days.The results showed that compared with the control,the hind limb lengths(HLL) in 0.01, 0.05 and 0.25mg·L-1 treatment groups were decreased by 17%,11%and 18%at 7 day after exposure,respectively,and development stages were significantly postponed;when test was terminated,the body weights(BW) in all treatment groups were increased by 17%,10%and 20%, snout to vent lengths(SVL) were increased by 5%,4%and 6%,respectively,and hind limb lengths(HLL) in 0.01 and 0.25mg·L-1 treatment groups were decreased by 12%and 13%, respectively.PFOS decreased the number of follicle,and induced thyroid follicle cell hyperplasia, the depletion of colloid and even vacuolation.In addition,amalgamation of two or more follicles occurred in 0.05 mg·L-1 treatment group.Our results suggested that PFOS in treatment dose could accelerate the growth of tadpoles and postpone their metamorphosis development.PFOS could have anti-thyroid hormone disrupting effects.Secondly,we used Xenopus completed metamorphosis assay(XCMA) to reveal endocrine disrupting effects of PFOS.X.laevis tadpoles(NF48) were exposed to PFOS at the concentrations of 0.01,0.1 and 1mg·L-1 for 6 months.The results showed that PFOS could accelerlate the metamorphosis of tadpoles at 0.01mg·L-1,malformations of gonad with intersex,pearling and segmental aplasia were found in all treatment groups,induce the abnormal elevation of sex ratio (female/male),and the ratio of abnormal gonad declined with increasing the concentrations of PFOS.The histological structures of thyroid gland and gonad were observed.PFOS induced thyroid follicle cell hyperplasia,the depletion of colloid and even vacuolation.And the gonad with intersex showed a testis tissue with a female ovarian cavity like structure.These results showed that PFOS could stimulate the metamorphosis of tadpoles at low dose,induce the damage of thyroid histology,and lead to the feminization of testis histology.In brief,PFOS had significant thyroid hormone and sex hormone disrupting effects.Additionally,these results suggested that X. laevis could be used as the model animal to test simultaneously two endocrine disrupting effects.Thirdly,we used prolonged Xenopus completed metamorphosis assay(prolonged-XCMA) to reveal endocrine disrupting effects of PFOS.X.laevis tadpoles were exposed to PFOS at the concentrations of 0.01 and 1mg·L-1 from NF48 to 1 month postmetamorphosis.The results showed that PFOS could restrain the development of tadpoles,delay the metamorphosis time,and induce thyroid follicle cell hyperplasia,the depletion of colloid and even vacuolation.The body length and body weight of NF66 Xenopus laevis were significantly increased in 1mg·L-1 treatment groups.PFOS could induce the asymmetry of ovary development and result in subsection, hypoplasia,elongated and narrow,aplasia and asymmetry of the testes,especially subsection which was the worst phenomenon.The female ovarian cavity in testis was very common.The ratio of male changed a little in control and treatment groups,but the ratio of abnormal gonad declined with increasing the concentrations of PFOS.These results showed that PFOS could restrain the metamorphosis of Xenopus laevis and have significant thyroid hormone and sex hormone disrupting effects.In addition,we used Xenopus juvenile assay(XJA) to test the sex hormone disrupting effects of PFOS.X.laevis juveniles(1 month postmetamorphosis) were exposed to PFOS at the concentrations of 1mg·L-1 for 6 months.The testicular histology structures were observed and there were no significant changes in the testicular microstructure exposed by PFOS.The observation by transmission electron microscope methods showed that PFOS could induce vacuolization in sperm nuclei edge and interior,vacuolization in mitochandria interior,anomaly of nuclear outline and rigidification in lipid droplets edge.These results showed that PFOS could damage the testicular ultramicrostructure,but the testicular microstructure had only a little effect.Finally,Xenopus tropicalis tadpoles were used to screen environmental endocrine disruptors and we made a preliminary study.Food screening assay was firstly tested.The endpoints of survival rate,metamorphosis rate,body length,body weight and developmental stages of Xenopus tropicalis tadpoles were assessed.The results showed that the NO.10 food(Sera vipan) was the most suited food for Xenopus tropicalis tadpoles.Screening environmental endocrine disruptors assay by Xenopus tropicalis was tested.This experiment was a good explore for sceening endocrine disrupting effects using Xenopus tropicalis as the model animal.In conclusion,these results of several assays indicated that PFOS could inhibit the metamorphosis development of tadpoles,induce the damage of thyroid histology,result in several malformations of gonad and induce the damage of testicular histology structures and ultramicrostructure.PFOS had endocrine disrupting effects for the thyroid system and reproductive system of amphibian.PFOS was a thyroid hormone disrupting chemical,but also sex hormone disrupting chemical and could be regarded as one of environmental endocrine disrupting chemicals.This paper provided morphological and histological evidences for proving the thyroid hormone and sex hormone disrupting effects of PFOS.Additionally,these results suggested that Xenopus could be well used as the model animal to test endocrine disrupting effects.
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