To quantify RIA-labeled cells in chick embryos, three consecutive sections of the same axial level were imaged per embryo. The number of YFP-positive cells was averaged to account for variability due to sampling. n = 4–6 embryos were analyzed at each stage as biological replicates. The results are presented as presence or absence of virally labeled cardiac neural crest derivatives at different anatomical locations in Figure 1I and as numerical values in Supplementary file 1a, 1b. To quantify Wnt1-Zsgreen+ cells in E15.5 mouse heart, three consecutive sections of the same axial level were imaged per embryo (n = 4). Automated particle analysis was conducted with FIJI program to estimate the total number of Zsgreen+ cells in the image. For the percentage of neural crest-derived cells in the ventricle, the same procedure was performed with the DAPI channel which represents total cell population. % Zsgreen/DAPI was calculated, and averaged to the result presented in the text of Supplementary file 1a. Same analysis was conducted to estimate the number of sox10:eGFP+ cells in 7dpa (n = 3), 21dpa (n = 3) and sham operated (n = 3) hearts in an area of 2 × 105 μm2 at the apex. One section per heart at the middle of the apex was quantified and presented in Supplementary file 1c.