Incidence and pathological features of spontaneous tumors in athymic nude mice.

Systematic observation of 1141 nude mice (Swiss background [corrected] strain) that received human tumor transplants revealed 24 spontaneous tumors, 18 of lymphoreticular origin and 6 pulmonary adenomas. Spontaneous tumors were seen at an average age of 9.1 months, and 22 of the tumors were seen only in that fraction of our group (324 mice) surviving for 5 months or more (22 of 324 X 100 = 6.8%). Transplantation of these tumors to other nude mice was successful in three of five cases. Mice transplanted with adenocarcinoma of the colon and with tumors of the urogenital tract developed spontaneous tumors more often than did mice receiving other types of human tumor transplants. Progressive growth of the human tumor transplant occurred significantly less often in the mice that eventually developed spontaneous tumors than in the mice that showed no spontaneous tumor development. Nevertheless, the incidence of spontaneous tumors in these nude mice was similar to that reported for the thymus-bearing background strain.