Practice question

Planarian regeneration requires rapid re-establishment of opposing Wnt and Notum domains at wounds. Early injury signals activate MAPK Erk within minutes via calcium and ROS, leading to phosphorylation of Elk transcription factors that bind notum promoter inducing its expression specifically at anterior-facing wounds. Erk activity simultaneously represses wnt1 transcription anteriorly. Consequently balance between Wnt-driven tail and Notum-driven head is orchestrated by wound-induced Erk gradient. Pax6 specifies eye differentiation, FGF10 mediates vertebrate limb outgrowth, Hedgehog upstream modulates wnt expression but immediate post-amputation balance controller is Erk signaling acting as wound orientation interpreter translating injury into polarized gene expression.