Axon regeneration by developing limb motoneurones in Xenopus laevis.

Newly generated motoneurones invading the limb bud were labelled with horseradish peroxidase to identify them later in development. They were then axotomized by amputating the limb bud. The limb bud was replaced immediately in some animals. After the period of normal motoneurone death, which falls a few days after the operation, many horseradish peroxidase labelled motoneurones were still alive provided the limb bud had been replaced. Without a limb bud, labelled motoneurones were few or non-existent. The conclusion is that in spite of their immaturity, the labelled motoneurones must have grown new axons into the limb for their death to have been avoided. Axon regeneration therefore seems to be inherent to motoneurones of any age.