Thyroid hormone regulates squamation onset and progression. (A) Timeseries of sp7:EGFP expressing scales showing single representative euTH and hypoTH individuals (n = 6, n = 5, respectively). In the euTH individual, squamation spread to the dorsal midline in four days, while this process took 20 days in the absence of thyroid hormone. (B) The number of scales increased rapidly in euTH fish until reaching the adult complement. In hypoTH fish, scales were added over a long period of time, leading to more total scales than in euTH (n = 6 euTH, 5 hypoTH). (C) Body surface area expanded dramatically during the slow squamation of hypoTH fish (n = 6 euTH, 5 hypoTH). (D) The distances between anterior–posterior neighbor scales were slightly, but significantly greater in hypoTH fish relative to euTH (F1,13.3 = 8.0 p = 0.014; n = 10 measurements from each of 6 euTH and 5 hypoTH individuals with individual variation treated as a random effect in mixed model ANOVA; distances were ln-transformed for analysis to control for increased variability at larger distances). Bars, means ± 95% CI. (E) Cartoon showing how delayed squamation coupled with a nearly normal interscale distance leads to increased scale density in adults. Scale bars, (A) 1 mm.
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