Fig. 1

Immortalization of primary zebra finch embryonic fibroblasts (ZEFs) by c-MYC expression. A The growth curve of primary ZEFs. Subculturing was performed on days 5, 10, 15, and 20. PDL, population doubling level. B Schematic illustration of the immortalization of primary ZEFs via retroviral vector-mediated transduction. C The morphology of c-MYC-treated cells. Scale bar, 100 μm. D The growth curve of c-MYC-treated cells. E Cellular senescence of primary cells and the immortalized cell line. The black arrowhead indicates a stained (scenescent) cell. Scale bar, 100 μm. SA‐β‐Gal, senescence‐associated-β-galactosidase. F The percentages of senescent primary and immortalized cells; **p < 0.01. G Cell cycle analyses of primary cells and the immortalized cell line. H Karyotyping analysis of primary cells at passage 1 and immortalized cells at passage 65. Karyotyping analysis only involved in the macrochromosomes of zebra finch. I Telomerase activity of the immortalized cell line at passages 1, 31, and 72. The gene expression levels of ectopic mouse c-MYC from the pMXs-c-MYC plasmid and endogenous zebra finch c-MYC, TERT, and RB1 were analyzed by RT-PCR. DW, distilled water