Differentiation of a methylcholanthrene-induced sarcoma to a benign plexiform fibroneural tumor in an adult frog (Rana pipiens). Possible influence of host regenerative capacity.

A spindle cell sarcoma appeared 20 months after implantation of a pellet of 3-methylcholanthrene in the denervated foreleg of an adult frog, Rana pipiens. Its growth rate and cellular structure were observed over the subsequent 19 months, the former remaining constant for the first 14 months, then slackening markedly during the final 4 months. Serial biopsies disclosed maturation to a well-differentiated fibroneural tumor of benign appearance, the change taking place notably during the period of decelerated growth rate. An autotransplant of the tumor to the hind leg adopted the same growth pattern and maturation. Possible causes for this behavior are discussed.