Thymidine labelling studies in a transmissible venereal tumour of the dog.

The cell population kinetics of the transmissible venereal tumour of the dog was studied at two different stages of tumour growth using the labelled mitoses technique. At the first stage the tumours were growing with a doubling time of about 4 days; at the second stage their growth rate was limited, probably by an immune reaction on the part of the host, to a doubling time greater than 20 days.Labelling of the tumour cells was found to be extremely heterogeneous throughout the tumour. Mitotic figures, however, were present in well labelled as well as in poorly labelled fields, suggesting that thymidine did not reach all regions of the tumour nodules. The data were therefore analysed assuming that the cells in well labelled areas were representative of the total cell population in the neoplasm. The timing of the cell cycle was found to be similar in the rapidly growing tumours and in those growing more slowly. It is concluded that the slowing of growth was due to a considerable increase in the rate of cell loss as a result of the immune reaction.