ToxSci Advance Access originally published online on January 4, 2006
Toxicological Sciences 2006 90(2):337-348; doi:10.1093/toxsci/kfj083
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Evaluation of Histological and Molecular Endpoints for Enhanced Detection of Thyroid System Disruption in Xenopus laevis Tadpoles
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* Department of Inland Fisheries, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, D-12587 Berlin, Germany; Aquatic Ecology and Toxicology, Department of Zoology, University of Heidelberg, D-69120 Heidelberg, Germany; and Department of Endocrinology, Institute of Biology, Humboldt University Berlin, D-10115 Berlin, Germany
Received August 24, 2005; accepted December 20, 2005
Amphibian metamorphosis represents a promising model for the identification of thyroid system–disrupting chemicals due to the pivotal role played by thyroid hormones for the initiation and regulation of metamorphosis. An important aspect of bioassay development is the identification and evaluation of sensitive and diagnostic endpoints. In this study, several morphological, histological, and molecular endpoints were evaluated for their utility to detect alterations in thyroid system function after exposure of stage 51 Xenopus laevis tadpoles to various concentrations (1.0, 2.5, 10, 25, and 50 mg/l) of the anti-thyroidal compound ethylenethiourea (ETU). Analysis of developmental stages on exposure day 20 and monitoring of time to fore limb emergence (FLE) revealed retardation and complete arrest of tadpole development at 25 mg/l and 50 mg/l ETU, respectively. Development was not affected by 1.0, 2.5, and 10 mg/l ETU. Histological alterations in the thyroid gland were observed in FLE-displaying tadpoles after exposure to 2.5, 10, and 25 mg/l ETU, as well as in developmentally arrested tadpoles exposed to 50 mg/l ETU. Prevalence and severity of histological changes increased in a concentration-dependent manner. Semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) showed increased mRNA expression of the 
- and ß-subunits of thyroid-stimulating hormone (TSH, TSHß) in pituitary tissue of tadpoles exposed to 25 and 50 mg/l ETU. Results demonstrate the successful detection of anti-thyroidal effects of ETU in Xenopus laevis tadpoles using various endpoints and highlight the particular sensitivity of thyroid gland histology to detect thyroid system disruption in tadpoles.
Key Words: Xenopus laevis; amphibian metamorphosis; endocrine disruption; thyroid system; histology; RT-PCR.
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