Effects of thyroid inhibition and rearing temperature on zebrafish development

Zebrafish development is a complex process that is highly dependent on the thyroid system. The purpose of this study was to examine the effects of thyroid hormone deprivation and rearing temperature on embryonic development. Embryos were exposed to different concentrations of methimazole (MMI), a known thyroid inhibitor, and maintained at either 28oC or 31oC. Changes in growth were assessed by determining changes in 4 parameters: eye size, inter-eye distance, body length, and spinal curvature. Initial experiments tested the effects of three MMI concentrations (0.1mM, 0,2mM, and 0.3mM) at three developmental ages (50, 60, and 72 hours postfertilization; hpf). Results from this study identified 60-72hpf as a critical developmental window for thyroid inhibitor sensitivity and 0.3mM as the MMI concentration that gave the most consistent results. Consequently, subsequent experiments exposed embryos to 0.3mM beginning at 0-1hpf. Embryos were removed from treatment hourly between 60 and 72hpf and placed in control water where they were allowed to develop until 72hpf, at which time they were euthanized and fixed for later analysis. Results of morphological measurements indicated a significant temperature effect for all growth parameters examined (p < 0.01), and that the embryonic thyroid system is functioning and capable of feedback mechanisms as early as 66 hpf. Further, we found that MMI significantly affected the development of retinal layers, where both temperature and MMI exposure caused significant effects at 72 hpf.