_ * . f HOW X RAYS BLOCK LIMBREGENERATIONIN SALAMANDERS AND A METHODFOR REINSTATING REGENERATION 5S. MERYL ROSE Tulane University, New Orleans, Louisiana ABSTRACT Evidence that limb regeneration falls in salamanders after X Irradiation because nerves fall to make proper contacts with the epidermis is reviewed. This failure to make proper contacts results in a failure of the flow of information between cells. X-rayed cells are capable of regeneration and take an active part in it if the communication system is restored by grafts of norma) tissue. It has long been known that X irradiation in the dosage range from 2000 to 7000 R prevents limb regeneration in salamanders,’ We are just beginning to understand how this is accomplished, Salamanders are the highest forms in the vertebrate sezies that can regenerate perfect limbs, If a distal part is removed, for example, the forearm and hand, the distal part of the stump transforms to the missing part, Any part of the limb can form the missing distal structures, When a part is missing, the distal part of the stump loses structure, i.e., fibers and skeletal matrix dissolve and muscle fibers break up. The remaining cells become embryonic in type. The blastemathat they compose resembles an embryonic limb bud and like a limb bud grows rapidly.? Within 12 days to a month after amputation, the new pattern of the missing part of the limb can be Seen. Clearly there must be some kind of communication between cells if they can respond to something missing and cooperate to regenerate the missing structures, Two observations indicate that X rays block regeneration by de- creasing communication between cells, First, ifa plece«w nb skin is grafted to the tail of a salamander and the tail ampv‘ated through the graft, the regenerate is a tail, If the tail is X-rayed before receiving 995