Participation of cartilage grafts in amphibian limb regeneration.

Irradiated axolotl arms bearing grafts of pure cartilage obtained from the non-irradiated humerus of other specimens show a variable response to amputation of the hand. Some arms grow spikes with little morphogenesis, others produce one or more digits, and some regenerate small and defective but recognizable hands. Identically irradiated control arms which carry no grafts or grafts of irradiated tissue show none of these features. The cartilage grafts must therefore be responsible for the limited amount of regeneration observed. The origin of the regenerated tissues could not be independently determined in this experiment, although grafts were made reciprocally between white and dark genotypes, as all the regenerates were either very pale or completely unpigmented. A previous demonstration that grafted tissue does not reactivate irradiated host cells, however, permits the conclusion that the grafted cartilage provides all the mesodermal tissues of these regenerates. The grafts consisted exclusively of chondrocytes, while the regenerates contained cartilage, muscle, blood vessels, nerve sheaths and general connective tissue. It is concluded that chondrocytes dedifferentiate into pluripotent blastemal mesenchyme cells which can redifferentiate into all other internal tissues of the limb. The concepts of modulation and neoblasts (undifferentiated reserve cells) thus have no basis in amphibian limb regeneration.