TH regulates dermal migration. (A) In-toto imaging of dermis in ET37:EGFP transgenics revealed a transition between a single-layered primary hypodermis to scaled skin over seven days [17–23 days post fertilization (dpf)]. Beginning at 17 dpf migratory dermal cells were apparent over the ribs (inset 1) but not above the anal fin (inset 2). A similar distribution was observed in hypoTH fish at 25 dpf. Dermal migration occurred in a wave that spread first posteriorly and then dorsally in both euTH and hypoTH fish, followed by scale papilla formation. Although the same qualitative sequence occurred in hypoTH individuals, it progressed more slowly. (B) Dermal migration was initiated in significantly older hypoTH fish compared with euTH controls (t19 = 9.19, p < 0.0001; n = 9 euTH, 12 hypoTH). (C) Owing to delayed initiation, body surface area was significantly larger in hypoTH fish at the onset of dermal migration relative to controls (t19 = 18.45, p < 0.0001). (D) Density of primary hypodermal cells at the onset of dermis migration was more variably and slightly increased in hypoTH fish compared with euTH controls t18 = 2.56, p = 0.02). (E) dSFCs differentiated in the superficial dermis in both euTH and hypoTH fish indicated by sp7:nucEOS expression (magenta). Bars in B–D, means ± 95% CI. Scale bars, (A) 500 μm for whole skin overviews or 100 μm for details, (E) 100 μm low magnification and 20 μm for details.
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