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Fig. 2 RA signaling is necessary for blastema formation. (A) Inhibition of RA signaling in hsp70:cyp26a1 fish by applying daily heat shocks (commencing 2 hours before fin amputation) results in an early and complete block to fin regeneration. (B-D) Hematoxylin-stained longitudinal sections indicate absence of blastema cells in hsp70:cyp26a1 fins at 45 hpa and lack of a distinctive basal epidermal layer. Several layers of epithelial cells seal the amputation plane, indicating normal initial wound healing. Remodeling of the stump mesenchyme adjacent to the amputation site is apparent in both wild-type and hsp70:cyp26a1 fish. (B) Overviews of stained sections. (C,D) Magnified view of the wound epidermis-mesenchyme boundary (C) and the stump mesenchyme (D). Dashed lines indicate amputation plane. hs, heat shock; b, blastema; be, basal epidermal layer; sm, stump mesenchyme; we, wound epidermis. Scale bars: 500 μm in A upper panels; 200 μm in A lower panels; 100 μm in B; 50 μm in C,D.