An examination of historical control histopathology metadata from 51 Amphibian Metamorphosis Assays

Comparing the effects and potencies of perchlorate and nitrate on amphibian metamorphosis using a modified amphibian metamorphosis assay (AMA)

A modified amphibian metamorphosis assay was performed in which Nieuwkoop and Faber (NF) stage 47 Xenopus laevis larvae were exposed to different concentrations of either perchlorate (ClO4 -) or nitrate (NO3 -) for 32 days. Larvae were exposed to 0.0 (control), 5, 25, 125, 625, and 3125 μg/L ClO4 -, or 0 (control), 23, 71, 217, 660, and 2000 mg/L NO3 -. The primary endpoints were survival, hind limb length (HLL), forelimb emergence and development, developmental stage (including time to NF stage 62 [MT62]), thyroid histopathology, wet weight, and snout-vent length (SVL). Developmental delay as evidenced by altered stage distribution and increased MT62, a higher degree of thyroid follicular cell hypertrophy, and an increase in the prevalence of follicular cell hyperplasia was observed at concentrations ≥125 μg/L ClO4 -. The no observed effect concentration (NOEC) for developmental endpoints was 25.0 μg/L ClO4 - and the NOEC for growth endpoints was 3125 μg/L ClO4 -. Exposure to nitrate did not adversely affect MT62, but a decreasing trend in stage distribution and median developmental stage at ≥217 mg/L NO3 - was observed. No histopathologic effects associated with nitrate exposure were observed. An increasing trend in SVL-normalized HLL was observed at 2000 mg/L NO3 -. Nitrate did not alter larval growth. The NOEC for developmental endpoints was 71 mg/L NO3 -, and 2000 mg/L NO3 - for growth endpoints. The present study provided additional evidence that the effects and potency of nitrate and perchlorate on metamorphosis and growth in X. laevis are considerably different.

Historical control histopathology data from amphibian metamorphosis assays and fathead minnow fish short term reproductive assays: A tool for data interpretation

The Amphibian Metamorphosis Assay (AMA) is used to determine if a tested chemical has potential to impact the hypothalamic-pituitary-thyroid (HPT) axis of Xenopus laevis tadpoles, while the Fish Short Term Reproduction Assay (FSTRA) assesses potential effects on the hypothalamic-pituitary-gonadal (HPG) axis of fish such as the fathead minnow (Pimephales promelas). Several global regulatory programs routinely require these internationally validated tests be performed to determine the potential endocrine activity of chemicals. As such, they are conducted in accordance with standardized protocols and test criteria, which were originally developed more than a decade ago. Sizeable numbers of AMA and FSTRA studies have since been carried out, which allows for the mining of extensive historical control data (HCD). Such data are useful for investigating the existence of outlier results and aberrant control groups, identifying potential confounding variables, providing context for rare diagnoses, discriminating target from non-target effects, and for refining current testing paradigms. The present paper provides histopathology HCD from 55 AMA studies and 45 fathead minnow FSTRA studies, so that these data may become publicly available and thus aid in the interpretation of future study outcomes. Histopathology is a key endpoint in these assays, in which it is considered to be one of the most sensitive indicators of endocrine perturbation. In the current review, granular explorations of HCD data were used to identify background lesions, to assess the utility of particular diagnostic findings for distinguishing endocrine from non-endocrine effects, and to help determine if specific improvements to established regulatory guidance may be warranted. Knowledge gleaned from this investigation, supplemented by information from other recent studies, provided further context for the interpretation of AMA and FSTRA histopathology results. We recommend HCDs for the AMA and FSTRA be maintained to support the interpretation of study results.

Effects of perchlorate and exogenous T4 on growth, development and tail resorption of Rana chensinensis

Endocrine disruptors have been demonstrated to exert adverse effects on growth and development of amphibians by disrupting hormone levels. Tail resorption, which is one of the most remarkable events during amphibian metamorphosis, is closely associated with thyroid hormones levels. However, limited research has been conducted on the effects of endocrine disruptors on tail resorption in amphibians. This study explored the effects of NaClO4 and T4 on the growth, development and tail resorption during the metamorphosis of Rana Chensinensis. The results demonstrated that exposure to NaClO4 led to an increase in body size and a delay in metamorphosis of R. Chensinensis tadpoles. Histological analysis revealed that both NaClO4 and exogenous T4 exposure resulted in thyroid gland injury, and NaClO4 treatment delayed the degradation of notochord and muscles, thereby delaying tail resorption. Moreover, transcriptome sequencing results showed that apoptosis-related genes (APAF1, BAX and CASP6) and cell component degradation-related genes (MMP9 and MMP13) were highly expressed in the T4 exposure group, and the expression of oxidative stress-related genes (SOD and CAT) was higher in the NaClO4 exposure group. Taken together, both NaClO4 and exogenous T4 affect tail resorption in R. Chensinensis, thereby affecting their adaptation to terrestrial life. The present study will not only provide a reference for future experimental research on the effects of other endocrine disruptors on the growth, development and tail resorption of amphibians but will also provide insights into environmental protection and ecological risk assessment.

An Amphibian Metamorphosis Assay Dietary Restriction Study: Lessons for Data Interpretation

The amphibian metamorphosis assay (AMA) is a key in vivo endocrine screen to investigate chemicals with potential thyroid activity. The test guidelines and associated guidance consider that treatment-related effects on thyroid gland histomorphology automatically result in the assay being considered positive for thyroid activity, independent of the direction of change or conflicting results in the other biological endpoints. An AMA study was conducted with five different feeding rations equivalent to 50%, 30%, 20%, 10%, and 5% of the recommended feeding rate. Biological endpoints relating to growth and development, including thyroid gland histopathology, were evaluated, and the specificity of these endpoints for the determination of thyroid activity was assessed. There was no effect on survival or clinical signs of toxicity. Effects related to feed reduction generally occurred in a feeding ration-response manner and included reduced development stage; reduced body weight and body length metrics; decreased prevalence of thyroid follicular cell hyperplasia and hypertrophy, and the occurrence of thyroid atrophy; reduced liver vacuolation; and the occurrence of liver atrophy. The results indicate that treatment-related histopathological changes in the AMA can be induced by Non-chemical factors; therefore histopathological results are not necessarily diagnostically specific for chemically induced thyroid endocrine activity. Consequently, the interpretation of data from AMA studies should be adjusted accordingly. We recommend that the decision logic presented in the test guidelines and associated guidance be changed to reflect a requirement for directional agreement between the thyroid histopathology findings and the growth and developmental endpoints before it is concluded that a test substance has thyroid endocrine activity. Environ Toxicol Chem 2023;42:1061-1074. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Influence of systemic copper toxicity on early development and metamorphosis in Xenopus laevis

The primary objective of the present study was to examine the influence of early systemic toxicity resulting from copper (Cu) exposure on metamorphic processes in Xenopus laevis. A 28-day exposure study with copper, initiated at developmental stage 10, was performed using test concentrations of 3.0, 9.0, 27.2, 82.5, and 250 μg Cu/L. The primary endpoints included mortality, developmental stage, embryo-larval malformation, behavioral effects, hindlimb length (HLL), growth (snout-vent length [SVL] and wet body weight), and histopathology. The 28-day LC50 value with 95% confidence intervals was 61.2 (51.4-72.9) μg Cu/L with 250 μg Cu/L resulting in complete lethality. Developmental arrest in the 82.5 and delay in the 27.2 μg Cu/L treatments was observed as early as study day 10 continuing throughout the remainder of exposure. SVL-normalized HLL, body weight, and SVL in the 27.2 and 82.5 μg Cu/L treatments were significantly decreased relative to control. At 82.5 μg Cu/L, and thyroid gland size was markedly reduced when compared with controls consistent with the stage of developmental and growth arrest. Concentration-dependent findings in the intestine, liver, gills, eyes, and pharyngeal mucosa were consistent with non-endocrine systemic toxicity. These were prevalent in the 9.0 and 27.2 μg Cu/L treatment groups but were minimally evident or absent in the 82.5 μg/L group, which was attributed to developmental arrest. In conclusion, developmental delay in larvae exposed to 27.2 and 82.5 μg Cu/L was the result of systemic toxicity occurring in early development prior hypothalomo-pituitary-thyroid axis (HPT)-driven metamorphosis and was not indicative of endocrine disruption.

Importance of diet in amphibian metamorphosis-based studies designed to assess the risk of thyroid active substances

The present study evaluated the hypothesis that dietary quality used in historical studies may impact the effects of chemical stressors on premetamorphic development and metamorphosis due to suboptimal nutritional quality. A modified Amphibian Metamorphosis Assay (AMA) was performed in which Nieuwkoop and Faber (NF) Stage 47 tadpoles of Xenopus laevis were exposed for 32 days to iodide (I^- )-deficient FETAX solution supplemented with <0.025, 0.17, 0.52, 1.58, and 4.80 μg I^- /L (measured concentrations 0.061, 0.220, 0.614, 1.65, and 4.73 μg I^- /L) and fed a pureed Frog Brittle (FB) diet. An AMA guideline benchmark group (four replicates) exposed to dechlorinated tap water and fed standard Sera Micron Nature® (SMN) diet was evaluated concurrently. Developmental delay, observed as changes in stage distribution or median developmental stage, occurred in FB treatments with 0.061, 0.220, and 0.614 μg/L I^- , respectively. Developmental rates and hind limb length of the 1.65 and 4.73 μg/L I^- groups were similar to each other, but both treatments fell short of the developmental rate achieved by the SMN benchmark. Iodide supplementation also had no impact on nonthyroidal growth endpoints, which were markedly reduced in FB-fed frogs compared with their SMN-fed counterparts. All larvae that received the FB diet had mildly to severely hypoplastic/atrophic thyroids, a condition for which iodine supplementation had little if any ameliorative effect. Collectively, these results suggested that nutritional deficiencies in the FB diet negatively affected both growth and metamorphic development, the latter of which was only compensated to a limited extent by iodine supplementation.

Adverse Outcome Pathways for Chronic Copper Toxicity to Fish and Amphibians

In the present review, we synthesize information on the mechanisms of chronic copper (Cu) toxicity using an adverse outcome pathway framework and identify three primary pathways for chronic Cu toxicity: disruption of sodium homeostasis, effects on bioenergetics, and oxidative stress. Unlike acute Cu toxicity, disruption of sodium homeostasis is not a driving mechanism of chronic toxicity, but compensatory responses in this pathway contribute to effects on organism bioenergetics. Effects on bioenergetics clearly contribute to chronic Cu toxicity with impacts at multiple lower levels of biological organization. However, quantitatively translating these impacts into effects on apical endpoints such as growth, amphibian metamorphosis, and reproduction remains elusive and requires further study. Copper-induced oxidative stress occurs in most tissues of aquatic vertebrates and is clearly a significant driver of chronic Cu toxicity. Although antioxidant responses and capacities differ among tissues, there is no clear indication that specific tissues are more sensitive than others to oxidative stress. Oxidative stress leads to increased apoptosis and cellular damage in multiple tissues, including some that contribute to bioenergetic effects. This also includes oxidative damage to tissues involved in neuroendocrine axes and this damage likely alters the normal function of these tissues. Importantly, Cu-induced changes in hormone concentrations and gene expression in endocrine-mediated pathways such as reproductive steroidogenesis and amphibian metamorphosis are likely the result of oxidative stress-induced tissue damage and not endocrine disruption. Overall, we conclude that oxidative stress is likely the primary driver of chronic Cu toxicity in aquatic vertebrates, with bioenergetic effects and compensatory response to disruption of sodium homeostasis contributing to some degree to observed effects on apical endpoints. Environ Toxicol Chem 2022;41:2911-2927. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Amphibian Metamorphosis Assay: Investigation of the potential effects of five chemicals on the hypothalamic-pituitary thyroid axis of Xenopus laevis

2-Ethylhexyl 4-hydroxybenzoate (2-EHHB), 4-tert-octylphenol (4-OP), 4-nonylphenol-branched (4-NP), benzyl butyl phthalate (BBP) and dibutyl phthalate (DBP) were evaluated using a 21-day Amphibian Metamorphosis Assay (AMA). Xenopus laevis larvae were exposed nominally to each chemical at 3.6, 10.9, 33.0, and 100 μg/L, except 4-NP concentrations were 1.8, 5.5, 16.5 and 50 μg/L. Endpoints included mortality, developmental stage, hind limb length (HLL), snout-vent length (SVL), body weight (BW), and thyroid histopathology. BBP and 4-OP accelerated development compared to controls at the mean measured concentration of 3.5 and 39.8 μg/L, respectively. An increase in developmental stage frequency distribution was observed for 4-OP at 39.8 and 103 µg/L, BBP at all concentrations and DBP at 143 µg/L. Normalized HLL was increased on study day (SD) 21 for all tested substances except 4-NP. Histopathology revealed accelerated development and mild thyroid follicular cell hypertrophy at all BBP concentrations, but moderate severity at 105 μg/L. Increased BW occurred for all chemicals except 4-OP. Increased SVL was observed for 4-NP, BBP and DBP on SD 21. There was insufficient evidence that 4-NP and 2-EHHB affected the hypothalamic-pituitary thyroid axis, however, BBP, DBP and 4-OP showed potential effects on amphibian metamorphosis and thyroid activity, albeit through different lines of evidence.