Detecting the effects of environmentally relevant concentrations of thyroid hormone disrupting compounds on amphibian development

Persistent organic pollutants such as PCBs have been hypothesized to contribute to the observed global decline of amphibian populations. Thyroid hormone (TH) disruption is one of the possible mechanisms for effects of xenobiotics on amphibian development. In addition to the important functions shared with other vertebrates, TH also has an important function in amphibians. The metamorphosis of amphibian larvae to froglet needs a TH surge shortly before the onset of metamorphosis to proceed. To study the potential disruption during two specific life-stages a bioassay with exposure during very early developmental stages and one for exposure of tadpoles just before the onset of TH dependent metamorphosis were developed. The assays were optimized, validated with PCBs as standards and applied for testing of realistic dosages of polluted sediment extracts. In addition, an in vitro assay using the TH dependent growth of a cell-line was established as a screening tool.The studies presented in this thesis reveal that the currently used early life stage test (FETAX) does not detect effects of the tested PCBs and apolar sediment extracts during the 96-hour test period whereas the newly developed prolonged-FETAX showed significant delayed effects of very low exposure concentrations on body weight, and on the period until- and percentage of animals finishing successful metamorphosis.In the so-called Synchronized Amphibian Metamorphosis Assay thiourea was used to synchronize tadpole development in NF stage 54 thereby starting experiments with a very homogenous group of animals. Significant effects of oral exposure to PCBs and apolar sediment extracts were found on the period until metamorphosis, and the distribution of the developmental stages of tadpoles that did not finish metamorphosis.The approach with exposing embryos (prolonged-FETAX) and tadpoles (Synchronized Amphibian Metamorphosis Assay) may finally better reflect environmental risks of apolar compounds than limiting the exposure to solely a single life-stage or water-exposure. The effects of the highly diluted apolar sediment extracts suggest the presence of TH disrupting compounds in the aquatic environment and possible effects of such compounds on development of amphibians and possibly other aquatic vertebrate species in the wild. The in vitro mode! showed its suitability and importance to study specific aspects of endocrine disrupting potency of toxic compounds