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Figure 6—figure supplement 1. Thyroid hormone (TH) drives dermal stratification via transcriptional regulation of epidermal Pdgfα expression. (A) Differential gene expression analysis between cell types of wild-type and hypoTH fish revealed extensive changes in expression across dermal and epidermal cell types (n = 836 genes, q-value < 0.01, normalized effect >2) (Supplementary file 2—Table 4). SB cell, suprabasal cell. (B) Of the differentially expressed genes, ligands of major signaling pathways involved in morphogenesis are also enriched in basal cells. (C) Both pdgfaa and pdgfab ligands are differentially expressed (***q-value < 1e-10) between wild-type and hypoTH basal cells of the epidermis (error bars estimated via bootstrapping [n = 100]). (D) Wild-type dermal and basal cells of epidermis plotted in UMAP space and colored by whether they express pdgfaa, pdgfab, or both (ligands) as well as pdgfra, pdgfrb, or both (receptors). (E) Upper left: mosaic heat-shock induction of Pdgfaa (magenta), stringently selected for expression in epidermal basal cells, rescued stratification of hypoTH dermis, visualized with ET37:EGFP (green) (n = 65 clones in eight fish). Upper right: higher magnification of boxed area showing accumulation of dermal cells underneath Pdgfaa+ epidermal cells. Bottom: optical cross section of boxed area reveals multiple dermal layers only in proximity to Pdgfaa+ epidermal cells. Scale bars, 50 μm (E, upper-left panel), 10 μm (E, enlarged region, upper-right and lower panels).