Repeated skin painting with 20-Methylcholanthrene (20 MC) caused lymphocytic leukemia in 54.2% of RF mice, of which 18.2% was of the thymic type and 36.3% of the non-thymic type.
The mode of development of the lymphocytic leukemia was serially studied. The peripheral blood pictures showed in the pre-leukemie stage a normochromic anemia, leukopenia, lymphopenia, eosinophilia, and reticulocytopenia; in the early leukemic stage, a slight leukocytosis with a few lymphoblasts; and in the progessive leukemiç stage, a hyperchromatic or normochomic anemia, anisocytosis, polychromasia, thrombocytopenia and leukocytosis with many lymphoblasts.
The imprint preparations disclosed an increase of lymphocytes in the lymphnodes and thymus in the pre-leukemic stage. In the early to progessive leukemic stage, there was a marked increase of lymphoblasts in the lymphnodes, spleen and thymus. In the bone-marrow, increare of lymphocytes and lymphoblasts were noted in the early to progessive leukemic stage.
Histological examination revealed a hypertrophie of the lymphfollicles and an increase of lymphoblasts in the spleen and lymphnodes in the initial leukemic stage of the non-thymic type leukemia. In the thymic type leukemia, a similar increase of lymphoblasts occarred in the thymus. In the early to progessive leukemic stage, an infiltration of lymphoblasts was seen in the spleen, lymphnodes, thymus and bone-marrow. In the liver, the infiltration was pronounced in the peripotal and perivascular spaces.
Tissue culture studies indicated a hypoplasia of the lymphnodes and thymus in the preleukemic stage. However, in the early leukemic stage, an acute leukemic growth pattern was exhibited by the spleen and lymphnodes in the non-thymic type while the thymus of the thymic type showed the same pattern. In the progressive leukemic stage, the spleen, lymphnodes, thymus and bone-marrow presented a culture pattern of acute leukemia.
The incidence of lymphocytic leukemia in splenectomiged RF mice painted with 20-Methylcholanthrene was 42.8%, and that of thymectomiged RF mice with a similar treatment was 18.2%.
From these finding, it is suggested that in RF mice painted with 20 MC the spleen, lymphnodes and thymus are the target organs which first manifest injurious effects and become hypoplastic and in the course of repair and regenerative process, hyperplastic changes untimately result in the leukemic proliferation in these organs.