[3H]thymidine labels less than half of the DNA-synthesizing cells in the mouse tumour, carcinoma NT.

We have studied carcinoma NT, a transplantable mouse adenocarcinoma of spontaneous origin. Cells labeled with [3H]thymidine ([3H]TdR) were restricted to a narrow zone around the periphery of this tumour and were also found in rings up to 50 micrometers wide, around isolated blood vessels in the central necrotic area. Labelling with [3H]deoxyuridine ([3H]UdR), another DNA synthesis precursor, produced a very different pattern. The labelled zone around the periphery was much wider than with [3H]TdR, and [3H]UdR labelled cells were found up to 110 micrometers from isolated vessels. [3H]iododeoxyuridine ([3H]IUdR) gave the same pattern of labelling as [3H]UdR. In the heavily labelled zone, within 1 mm of the tumour periphery, the labelling index (LI) was 51% after [3H]UdR or [3H]IUdR injection, and only 36% with [3H]TdR. The data show that at least half of the DNA-synthesizing cells in this tumour did not incorporate [3H]TdR. Previous workers reported cell loss factors for carcinoma NT of 60% calculated from [3H]TdR labelling data and 30% from the rate of loss of [125I]UdR. The present work suggests that calculations based on [125I]UdR data are more likely to be accurate for carcinoma NT than those using [3H]TdR data.