Rodent leukemia: Recent biological studies. A review.

Understanding of the factors influencing the spontaneous development and the transplantation of experimental neoplasms has been sought in studies of rat and mouse leukemia. Since leukemia is induced readily in various strains of mice by either carcinogenic hydrocarbons, ionizing radia tions, estrogens, or combinations of these agents (81, 66), the disease has been the subject of recent studies on experimental carcinogenesis. Because of their sensitivity to so-called “therapeutic― chemi cal agents, the leukemias of mice are being used extensively in screening programs (16) and for the study of cellular refractoriness to agents such as the folic acid antagonists (19, 7@). Morphological types.—The usual type of sys temic leukemia is lymphoid. Myeloid leukemia, often chiorotic, appears relatively infrequently (67). Spontaneous mediastinal (thymic) lympho sarcoma occurs primarily in young mice (49, 67). Unusual distribution of leukemic infiltrations may characterize the disease in hybrids, the patterns of disease being more uniform in the pure strain (49). In “high-leukemia― strains the incidence may be as high as 90 per cent, the majority of cases appearing by 1 year of age. Leukemia occurs spon taneously iii old animals, beyond 500 days of age,