Proliferating ERG1-positive glial cells give rise to neurons in the regenerating RNC. (A) BrdU labeling paradigm employed to label proliferating glial cells and trace their progeny. Multiple BrdU injections (50 mg/kg, every 12 hours) were given during the growth phase of regeneration (days 8 thru 12 post-injury). The tissues were fixed at two time points: 12 hours and 51 days after the last BrdU injection (on day 13 and day 64 post-injury, respectively). (B) Proportion of ERG1-positive glial cells among BrdU-positive cells. Note that shortly after BrdU administration during the growth phase, the vast majority of BrdU-incorporating cells show ERG-positive glial phenotype. This number significantly decreases on day 51 after the last BrdU injection, when ERG1-negative cells (neurons) represent almost half of the BrdU-positive progeny. (C, C’) Representative micrographs showing BrdU-incorporating ERG1-positive radial glial cells (arrowheads) 12 h after the last BrdU injection. (D, D’) Representative micrographs showing Nurr1+ BrdU+ neurons (arrows) on day 51 after the last BrdU injection (day 64 post-injury).