Developmental toxicity of short-chain chlorinated paraffins on early-stage chicken embryos in a shell-less (ex-ovo)...

Short-chain chlorinated paraffins (SCCPs) are listed as a category of globally controlled persistent organic pollutants (POPs) by the Stockholm Convention in 2017. However, SCCP toxicity, particularly their developmental toxicity in avian embryos, has not been well studied. In this study, we observed the early development of chicken embryos (Gallus gallus domesticus) by applying a shell-less (ex-ovo) incubation system developed in our previous studies. After exposing embryos at Hamburger Hamilton stage (HHS) 1 to SCCPs (control, 0.1% DMSO; SCCPs-L, 200 ng/g; SCCPs-M, 2000 ng/g; SCCPs-H, 20,000 ng/g), we observed the development of embryos from the 3^rd to 9^th incubation day. Exposure to SCCPs-M and -H induced a significant reduction in survival, with an LD₅₀ of 3100 ng/g on the 9^th incubation day. Significant dose-dependent decreases in body length were observed from days 4-9. We also found that SCCPs-H decreased the blood vessel length and branch number on the 4^th incubation day. Additionally, SCCPs-H significantly reduced the heart rate on the 4^th and 5^th incubation days. These findings suggest that SCCPs may have potential of developmental and cardiovascular toxicity during the early stages of chicken embryos. Quantitative PCR of the mRNA of genes related to embryonic development showed that SLC16A10 (a triiodothyronine transporter) level decreased in the SCCPs-H group, showing a significant positive correlation with the body length of embryos. THRA level, a thyroid hormone receptor, was significantly decreased in the SCCPs-H group, whereas that of DIO3 level, a deiodinase was significantly increased. These results suggest that SCCPs exposure induces developmental delays via the thyroxine signaling pathway. Analysis of thyroid hormones (THs) in blood plasma also indicated a significant reduction in thyroxine (T4) levels in the SCCPs-H group on the 9^th incubation day of embryos. In conclusion, SCCPs induce developmental toxicity by disrupting thyroid functions at the early-life stage of chicken embryos.