Cell kinetics of urethane-induced murine pulmonary adenomata: II, the growth fraction and cell loss factor.

Continuous labelling of urethane induced pulmonary adenomata in adult male A2G mice at intervals up to 20 weeks showed that the growth fraction fell progressively from 18% at 7 weeks to 7% at 20 weeks. This fall appears to be wholly responsible for the decrease in production of adenoma cells with age. A fraction labelled mitoses curve was constructed for pulmonary adenomata at 14 weeks post urethane. Only the first peak was apparent, giving median t2 and ts values of 2 and 9 h respectively. The cell cycle time was calculated at 45 h and the growth fraction at 6-2%, whilst the cell loss factor was estimated at 31%. Other cell loss values were calculated from data on the rate of entry into DNA synthesis obtained previously by double labelling. These values remained constant with time at 83-95%, suggesting that cell loss is in some way linked to cell production. However, the development of polyploidy in adenoma cells could not be eliminated. No areas of necrosis were seen in the adenomata at any time although karyorrhexis occurred in isolated cells. The labelling characteristics of alveolar wall cells in the same lung sections as the adenomata did not vary with time and the continuous labelling curve gave a growth fraction of 1-8%, a DNA synthetic time of 10 h and a cell cycle time of 30 h

Cell kinetics of urethane induced murine pulmonary adenomata: I. The growth rate.

A single injection of urethane into adult male A2G mice produced an increase in the proliferative rate of alveolar wall cells, reaching a peak at 2 weeks post urethan (PU) and declining to control levels by 2 months PU. During this urethane induced proliferative response the single and double labelling indices and the native metaphase index were all elevated although there was no corresponding alteration in the arrested metaphase index. This proliferative response may not be restricted to hyperplasia of potentially neoplastic cells, such as type II epithelium, but may also include type I epithelial cells and alveolar macrophage precursors. However, it was impossible to identify individual cell populations by methods used. The growth rate of adenomata decrease with time and cell kinetic techniques showed that the rates of entry of adenoma cells into DNA synthesis and into metaphase were decreasing concurrently with the growth rate. Thus the rate of cell production falls as adenomata age but how much cell loss contributes to the decrease in growth rate is not yet known. Decreasing cell production could be due to an increased cell cycle time and/or a decreased growth fraction. The duration of DNA synthesis in adenomata increased markedly as the mice survived, suggesting that the cell cycle time might be increased, but further experiments are required to determine whether the growth fraction changes. Attention is drawn to a complication that Colcemid introduces into kinetic studies on alveolar wall cells

Incidence of lung tumours induced by urethane in mice exposed to reduced atmospheric pressure.

INBRED mice of strain A are well known to possess a high incidence of pulmonary adenomas whether spontaneous or induced, whereas strain C57B1 mice show a very low incidence, which according to Bloom and Falconer (1964) is genetically determined. In attempting to overcome this natural resistance of C57B1 mice to tumour development several observations led us to vary the oxygen tension in which mice were maintained. Heston and Pratt (1956, 1959) employed high or low oxygen tensions in a chamber for two days and found respectively increased or diminished incidences of dibenzanthracene-induced lung tumours in strain A mice. On the other hand, Mori-Chavez (1962b) observed an increase in the number and in the size of urethane-induced tumours in strain A mice maintained at a natural high altitude for eight or more months, and Heppleston and Simnett (1964) found that elevated oxygen tension produced deleterious effects on lung tissue and pulmonary adenomas from high incidence strains maintained in organ culture. Strains of mice susceptible and resistant to tumour induction by urethane were therefore exposed for prolonged periods to low atmospheric pressur