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Fig. 3 Induction of cell proliferation in the fin stump requires RA signaling. (A-C) Inhibition of RA signaling in hsp70:cyp26a1 and hsp70:dn-zrar fish results in a significant decrease in proliferating cells in the fin stump (daily heat shocks, commencing 2 hours before amputation). (A) Longitudinal sections stained for BrdU and with DAPI demonstrate a near absence of BrdU-positive cells in hsp70:cyp26a1 fins at 32 hpa. (B,C) Quantification of BrdU-labeled cells within a defined area at 32 hpa in hsp70:cyp26a1 (wild type, n=39 sections; hsp70:cyp26a1, n=44) (B) or hsp70:dn-zrar stumps (wild type, n=20; hsp70:dn-zrar, n=31) (C). (D) Exogenous RA promotes mesenchymal proliferation in the stump [RA intraperitoneal injection (IP) every 12 hours, first IP at 0 hpa]. Quantification of BrdU-labeled cells at 32 hpa in RA-treated stumps (DMSO vehicle, n=16; RA, n=18). Error bars, s.e.m. *, P<0.0001 in B,C; *, P<0.005 in D. Dashed lines indicate the amputation plane. hs, heat shock. Scale bar: 100 μm.