CHROMOSOMES IN MURINE PRE-LEUKAEMIA 85 



the case of this last leukaemia from a propositus, as well as a clone of 39 in the 

 thymus, cells containing 39 chromosomes predominated in the bone-marrow. 

 The validity of the 39 clone in the thymus was verified by the transplantation 

 of the bone-marrow into a lethally irradiated C57BL. Twenty days later the 

 resultant chimaera was sacrificed with gross leukaemic involvement of spleen 

 and lymph nodes. Cells containing 39 chromosomes now predominated in 

 thymus, lymph node and bone-marrow. The findings are unusual since a 

 mouse leukaemia with a mode of 39 has not before been recorded. 



In addition to the 12 propositi, 13 radiation-induced leukaemias were 

 cytogenetically observed in the earliest possible passage. Of the total 25 

 leukaemic entities, 21 showed abnormalities of number and/or form. Two 

 had modes of 40 with clones of 39 and it is preferred to consider these clones 

 as spurious as re- examination of these leukaemias could not be made. Two 

 more have to be considered as not showing chromosome abnormalities 

 although bone-marrow and spleen and bone-marrow respectively in which no 

 abnormality was detected were the only tissues examined, since technical 

 failure was experienced with the thymic samples. 



Chromosomes in pre-leukaemia 



Against this background of chromosomal changes it was decided to 

 sample mice in the pre-leukaemic phase for evidence of karyotype alteration 

 in an attempt to elucidate which appeared first — the chromosomal anomaly 

 or the malignant change. Pre-leukaemia for the purposes of this experiment 

 has meant the state before macroscopic evidence of leukaemic involvement of 

 the thymus or other organs commonly involved. 



Batches of C57BL and C6 mice were subjected to four divided doses of 180 

 rads of ^°Co y- irradiation at 4-day intervals to a total whole-body dose of 720 

 rads. For C57BL and C6 incidence of the thymic type of leukaemia was 70% 

 and 68% with a mean onset expressed in days following the last irradiation of 

 222 and 202 respectively. All animals were examined in the period prior to 

 the mean time to onset. The thymus was not considered macroscopically 

 abnormal in size until a weight greater than 70 mg was recorded. Thus a 

 thymus neoplastic to the naked eye was excluded. 



Table 1 represents the cytogenetic analyses of thymus as well as bone- 

 marrow, spleen and lymph node from pre-leukaemic mice. The first impres- 

 sion is the number of abnormalities to be found in this class of material. From 

 the experience of observation of many thousands of normal cells of thymus, 

 bone-marrow, spleen and lymph gland in scoring the presence of the T6 

 marker in radiation chimaerism, the presence of these abnormalities of 

 structure and particularly number in pre-leukaemia is indeed striking. From 

 the laboratory's experience in counting the chimaera class of normal tissue 

 it has been somewhat arbitrarily considered that a clone is present if there are 



