Karyotypic variation of cells from murine thymic lymphomas during short-term culture

The present study examined the balance in the proportion of cells with either trisomy 15 or a diploid chromosome complement during short-term culture in vitro. Neonatal CFW/D mice were given an intrathymic injection of an endogenous virus (DMBA-LV) isolated from a DMBA-induced thymic lymphoma. The thymus was removed at the terminal leukemic stage, and the cells from each enlarged thymus were cultured in quintuplet. At 0, 6, 12, 24, and 48 hr, cultures were given colcemid for 2 hr prior to harvesting for chromosome examination. The analyzed lymphomas can be divided into three groups according to the characteristics of their stem lines and sidelines, as revealed by direct chromosome preparation (0 hr). Two groups consisted of lymphomas carrying a stem line with either trisomy 15 (group 1) or a diploid karyotype (group 2). A third group (group 3) consisted of lymphomas with a mixture of an abnormal stem line and sidelines. Chromosome analysis of cultures at subsequent times showed that there were fluctuations in the proportion of cells characterized as either stem line or sideline in all tumors at different times during culture. The initial sideline in group 1 tumors had a diploid complement. Although the proportion of the diploid cells increased in 5 of the 6 tumors following either 24 or 48 hr in culture, cells with a diploid karyotype remained as a sideline throughout the culture period. Similarly, the two tumors analyzed in group 2 showed that the initial sideline that contained trisomy 15 increased temporarily to become the stem line following 6 or 12 hr in culture. Group 3 tumors showed a slight variation in the proportion of cells with an abnormal stem line or sideline during culture. Considering all the data from the 10 tumors, 7 tumors had cells with a diploid karyotype, and, although the proportion of cells with this karyotype increased (4 tumors) or persisted (3 tumors) during 48 hr in culture, it remained as a sideline and was not overgrown by trisomy 15 stem line.