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

Abstract. We have studied carcinoma NT, a transplantable mouse adenocarcinoma of spontaneous origin. Cells labelled 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 μ m 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 μ m 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.