Proliferative Activity of Leukaemic Cells at Various Stages of Acute Leukaemia of Childhood

The DNA synthesis and content of individual leukaemic cells have been measured respectively by in‐vitro labelling with tritiated thymidine and micro‐densitometry of Feulgen‐stained cells. These investigations have been carried out on bone‐marrow and blood samples obtained from children at the time of diagnosis of acute leukaemia, and during subsequent remissions and relapses. At diagnosis, the labelling index of bone‐marrow blasts was low (6.8 ± 3.51 per cent), and that of blasts in the peripheral blood was usually still lower; most cells were at the 2n mode. There was no correlation between the labelling index and either the numbers of blasts in bone marrow or blood, or the duration of symptoms. During induction of remission, labelled cells disappeared more rapidly from the blood than unlabelled cells. In relapse, the labelling index was significantly higher than at diagnosis, and there was evidence of aneuploidy in most patients, as shown by the distribution of DNA content. The mean labelling index of the small proportion of blasts in the marrow of five patients during remissions was intermediate between those found at diagnosis and during relapse. The labelling index of leukaemic cells with tritiated uridine was high both at diagnosis and during relapse, the most heavily labelled cells usually being amongst the largest members of the population. The possible significance of these findings is discussed in relation to the natural history of the disease.