(A, B) FISH/immunofluorescence were used to determine cardiomyocyte specific expression (MyHC) of miR-99/100, Fntβ and Smarca5 in uninjured and regenerating zebrafish hearts (3 and 7dpa). Cardiomyocytes in regenerating hearts exhibited low levels of miR-99/100, and inversely correlating high levels of Fntβ and Smarca5 (n=8). (C–D) Immunofluorescence analysis demonstrating high levels of Fntβ (C) and Smarca5 (D) alongside markers indicative of proliferation, PCNA (C) and H3P (D), in dedifferentiating cardiomyocytes (n=5). (E–F) Quantitative analysis confirming the significantly higher number of MyHC+ cardiomyocytes co-expressing Fntβ/PCNA (E) and Smarca5/H3P (F) in the regenerating zebrafish heart (n=5 animals, three different sections per animal). Dashed line: amputation plane. Boxed area: magnified field. Arrows indicate cells of interest. Data are represented as mean ± s.e.m. *p<0.05. See also Figure S2 and S3.