An examination of the effects of Methyltriclosan on early embryonic development in the South African clawed frog (Xenopus laevis)
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Triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) is a commonly used bactericide present in many personal care products such as liquid detergents, liquid hand soaps, deodorants, cosmetics, creams, lotions, mouthwash, and toothpaste. Triclosan also can be added to fabrics, plastics, carpets, plastic kitchenware, and toys. Methyltriclosan is a derivative that is formed from triclosan via biological methylation at an unknown interval during waste water treatment. Methyltriclosan is more abundant in the environment, more lipophilic than triclosan, and has a greater potential to accumulate in fatty tissues. The global decline of amphibian populations has raised awareness surrounding the possible effects of poor water quality on the health of habitats. Since metamorphosis and reproductive development in amphibians is highly regulated by thyroid hormone (TH), and the structure of triclosan is similar to that of TH, there is the possibility that methyltriclosan may act on TH receptors to alter development. The response of Xenopus laevis embryos to various concentrations of methyltriclosan were tested in standard 96-hour Frog Embryo Teratogenic Assay-Xenopus (FETAX) protocols. After the 96-hour FETAX exposure, the larvae were photographed and total body lengths, snout-to-vent lengths and crown widths were measured using image analysis. The data revealed that early embryonic exposure to environmentally relevant concentrations of methyltriclosan resulted in significant (p<0.05) alterations in all of the measurements made that differed from the concentration-related response to triiodothyronine. Furthermore, molecular studies were performed in whole body tissues of the 96-hour-exposed larvae to identify the effects of methyltriclosan exposure on the TH-responsive gene, thyroid hormone bZIP. The results of the quantitative real-time polymerase chain reaction did not support the induction of TH/bZIP gene expression after exposure to environmentally relevant concentrations of methyltriclosan. We conclude that low environmental concentrations of methyltriclosan alter somatic growth in embryonic X. laevis through a mechanism that does not involve the activation of thyroid hormone receptors. Collectively, these data are the first to report on the responsiveness of vertebrate embryos to methyltriclosan exposure.