Comparing the effects of stage and duration of XXXX exposure on amphibian limb development: chronic exposure results in mortality, not @entity4218 .

@entity1238 @entity4878 @entity4218 @entity600 @entity854 @entity1968 @entity554
Recently, high frequencies of @entity600 have been reported in amphibians across the United States. It has been suggested that the @entity600 may be the result of xenobiotic disruption of @entity554 signaling pathways during embryogenesis and tadpole development. Therefore, a series of experiments were undertaken to examine life-stage specific effects of continuous @entity554 exposure on @entity1238 . Continuous @entity1968 (RA) concentrations were delivered using a column saturator and a flow-through diluter system. Stage 8 embryos were exposed to RA concentrations ranging from 0.013 to 2 microgram/l. At the onset of hindlimb bud emergence (NF stage 48), a subset of tadpoles was moved to clean water, and remaining organisms were exposed continuously through metamorphosis. In addition, early limb-bud-stage tadpoles were exposed for 1 week, 2 weeks, or until tail resorption was complete, to eight concentrations of RA in the range of 0.031-3 microgram/l. RA exposure resulted in a concentration-dependent increase in mortality and dysmorphogenesis in embryos at concentrations of 0.24 microgram/l and above. However, this early embryonic exposure did not result in @entity854 in surviving tadpoles allowed to mature in clean water. RA did not induce @entity4218 in any surviving tadpole exposed during larval stages. We are confident that the concentrations used were high enough, given that the highest concentration used resulted in 100% mortality within 2 weeks of initiating the exposure. This result suggests that other aspects of growth and development, which are not externally obvious, are more sensitive to @entity4878 than skeletal development. From these experiments and our previous work, we conclude that it is unlikely that @entity554 mimics would produce the spectrum of @entity4218 which recently have been observed in amphibians collected from the field.
