AbstractThe ultrastructural changes in the medullary canal of the adult male New Zealand white rabbit associated with the trauma of the insertion of an inactive cathode were compared to that associated with electrically induced osteogenesis from an active cathode delivering 20 μA constant direct current. In the vicinity of both an inactive and an active cathode, the medullary canal cellular content was replaced first by polymorphic cells and later by osteoblastic new bone formation. The polymorphic cells always appeared in the immediate vicinity of a capillary or other blood vessel. With time, the new bone formation in the medullary canal surrounding the inactive cathode disappeared, while new formation surrounding the active cathode progressively increased in amount. When the new bone formation in the medullary canal surrounding an inactive cathode disappeared, it was replaced by a network of polymorphic cells. When direct current was then applied to such an inactive electrode, the polymorphic cells were again rapidly replaced by bone. The close association of polymorphic cells with osteoblasts suggests that the former may be a precursor cell of the latter