Anurans can only regenerate their limbs during embryonic development. Once the limb skeleton has developed regeneration does not occur (Xenopus can grow a cartilaginous spike after amputation). Reactive oxygen species (ROS) appear to be required for a regeneration response in the anuran larvae. ROS production is essential to activate the Wnt signaling pathway, which has been associated with regeneration in other systems. Limb regeneration in salamanders occurs in two major steps. First, adult cells de-differentiate into progenitor cells which will replace the tissues they are derived from. Second, these progenitor cells then proliferate and differentiate until they have completely replaced the missing structure.