library_construction_protocol: Nuclei were prepared according to the 10X Genomics Chromium single-cell ATAC-seq solution protocol (Nuclei Isolation from Tissue for Single Cell ATAC Sequencing). In brief, amputated heads or blastema were transferred to 1.5 mL tubes, resuspended in 1xLysis Buffer and chopped to small pieces, and lysed on ice for 5 min. Lysed nuclei were diluted into the Wash Buffer and passed through a 40 µm filter. Nuclei was collected by centrifugation at 500 g for 5 min, resuspended in the Diluted Nuclei Buffer, and counted. Finally, nuclei were loaded on a Chromium Controller targeting 7,000 nuclei per sample. ScATAC-seq libraries were prepared for sequencing following the 10X Genomics single-cell ATAC–seq protocol and sequenced using PE50 sequencing on an Illumina NovaSeq 6000.