| THs play an important regulatory role in many physiological processes of fishsuch as the growth, sex differentiation, reproduction, osmotic regulation, and energymetabolism. Especially during the early development of fish, the lack of THs canaffect various morphogenetic and physiological development processes.Polychlorinated biphenyls (PCBs) have been listed as one of the twelve persistentorganic pollutants (POPs) under the Stockholm, which were also some of the mostwidely studied thyroid-disrupting chemicals (TDCs). Recently, more attention hasbeen paid to the disturbance of thyroid system by PCBs in fish. However, most fishthyroid disruption test used adult fish, which is more tolerance to contaminants,instead of larvae and juvenile fish in early development stage. Moreover, differentstudies are difficult to compare because of no accepted model organism in fish thyroiddisruption studies is established, hindering to figure out the exact mechanism of fishthyroid disruption caused by PCBs and the screens of TDCs in Aquatic environment.This study used commercialized PCBs mixtures, Aroclor1254to figure out thethyroid disruption effect of environmental related concentration of PCBs (10、100、1000ng/L) on juvenile Japanese flounder,testing histopathological, plasma THs level(total T3, T4level and free T3, T4level) and mRNA expression level of threedeiodinase (IDs, ID1, ID2and ID3). Meanwhile, we studied the influence of PCBs onjuvenile Japanese flounder metamorphosis and explained the mechanism of whyPCBs influenced metamorphosis by thyroid hormones, which is the key hormone inmetamorphosis regulate.The results of juvenile thyroid disruption showed that:(1) After25days of exposure, Aroclor1254significantly reduced WTand LTinall treatments. After50days of exposure,10ng/L and100ng/L Aroclor1254did not affect WT, but significantly inhibited LT. Exposure to1000ng/L Aroclor1254for50days significantly reduced WT, LT, and CF, relative to the control.(2) In flounder exposed to different concentrations of Aroclor1254for25days,the TT3, FT3, and FT4levels in the plasma were not significantly altered by any of thetreatments, whereas plasma TT4levels significantly decreased in the1000ng/L group.After50days of Aroclor1254exposure, both plasma TT3and FT3levels significantlydecreased in the1000ng/L group, with plasma TT4levels showing a dose-dependentdecrease, which was significant at concentrations of100and1000ng/L, while plasmaFT4levels remained unaltered.(3) For the quantitative analyses, significantly increased levels of follicularepithelial cell height, hyperplasia, and colloid depletion were observed in the100and1000ng/L Aroclor1254treatments.(4) The expression level of ID2mRNA in the liver and kindey exhibited adose-dependent increase, indicating that this ID2might serve as sensitive and stableindicator for thyroid-disrupting chemical (TDC) exposure.The effect of Aroclor1254on larvae metamorphosis and THs level:(1)79%of the larvae in control group entered metamorphic climax at24dph,and all larvae completed metamorphosis at37dph. For group100ng/L,83%of thelarvae were premetamorphosis at24dph,59%of the larvae were at metamorphicclimax at28dph. To the end of the experiment, only38%of the larvae completedmetamorphosis. In group1000ng/L, the inhibition of Aroclor1254on themetamorphosis of Paralichthys olivaceus was more severe. At26dph,60%of thelarvae were premetamorphosis,52%of larvae were at the stage of premetamorphosisat30dph, and60%of the larvae enter metamorphic climax at33dph. To the end ofthe experiment,42%of the larvae were still at metamorphic climax, and only21%ofthe larvae completed metamorphosis.(2) T4and T3levels were significantly elevated at metamorphic climax forcontrol group and exposure group10ng/L; while in groups100and1000ng/L, thetime when T4and T3levels elevated was delayed in comparison to control group.Particularly for exposure group1000ng/L, T4and T3levels in vivo at metamorphic climax were significantly lower than those of the same period in control group.(3) The control fish presented oval thyroid follicles of variable sizes that werefilled with colloid. In addition, the follicles were line with a single layer of cuboidal toflat follicle epithelial cells. Representative histopathological abnormalities in larvalflounder exposed to1000ng/L of Aroclor1254for41days, including increasedepithelial cell height, colloid depletion, and tended toward smaller follicular size.We studied the juvenile Japanese flounder disruption of thyroid and found thatenvironmentally relevant concentrations of PCBs exposure cause significant thyroiddisruption, conforming that juvenile fish are more sensitive than adult fish. We alsoconformed PCBs could inhibit metamorphosis of Japanese flounder by disruptingthyroid. This is the first report to document that exposure to TDCs at environmentalconcentrations inhibits metamorphosis in fish. Fish metamorphosis is regulated byTHs uniquely, so combine metamorphosis with other regular indicator could be moreexactly and generally to analyse the effect of contaminant on fish thyroid disruption. |
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