Injury requirement for initiation of regeneration of newt limbs which have whole skin grafts

Abstract

THERE are three known requirements for salamander limb regeneration—injury, nerves, and wound epidermis^1,2. During the past few years we have examined cellular events in amputated, regenerating limbs and compared these with non-regenerating limbs after either denervation^3,4 or after whole skin grafts over the amputation surface⁵. Results of these experiments led to the formulation of an hypothesis⁶ which suggests unique roles for injury, nerves, and the wound epidermis during the initiation of regeneration : injury is important to cause dedifferentiation of limb stump tissue cells, entry of these cells into G₁ of the cell cycle and DNA replication; nerves are important for one or more G₂ events and thus indirectly for mitosis; and the wound epidermis maintains these cells in the cell cycle so that significant outgrowth (blastema formation) can occur. In the present study we have examined the extent and kinds of injury necessary to initiate regeneration in newt limbs which have had skin grafts for 5 weeks and externally show no regeneration. The results suggest that non-cycling dedifferentiated cells are present in 5-week skin graft limbs which immediately begin cycling when a wound epidermis forms after re-injury.