Axolotl tail regeneration emerges during a defined embryonic window

How regenerative capacity originates during development remains poorly understood, even in vertebrates with exceptional adult regenerative ability. Using the axolotl, we identify a defined embryonic window between stages 30 and 34 during which the tail region transitions from a regeneration-incompetent to a regeneration-competent state. Amputations across staged embryos reveal that earlier embryos entirely fail to regenerate, whereas later embryos regenerate functional tails. Notably, tail stumps from nonregenerating embryos can recover the ability to regenerate when reamputated at later stages, demonstrating that early regenerative failure does not permanently impair regenerative capacity. This differs from the transient refractory period described in Xenopus, where regenerative competence is lost and reacquired around the end of tail outgrowth, and indicates that staged acquisition of regenerative competence is a broadly shared but mechanistically distinct feature of amphibian development. To determine whether this transition reflects changes in progenitor composition, we analysed the single-cell transcriptional landscapes of axolotl tail buds across this window. Tail bud progenitors, including neuromesodermal progenitors, persist through the transition, indicating that the onset of regenerative competence is unlikely to be explained by the loss of embryonic progenitors. Finally, using Tbxt (Brachyury) crispant axolotls with severe axial defects, we show that tail regeneration occurs effectively despite earlier abnormal embryonic tail development, with functional uncoupling of the mechanisms of tail development and regeneration. This framework provides new opportunities for identifying the drivers of regenerative competence and understand why this capacity is lost in other vertebrate species.