Proliferation of type II pneumonocytes after X-irradiation

Preliminary data are presented on the proliferative response of type II cells in mouse lung five-months after external thoracic doses of 2, 5, 10 and 12 Gy of X-rays. The DNA labelling index (LI) of control (O Gy) mice was at all times 0.3-0.4%. The LI after 2 and 5 Gy showed a slight though transient fall below controls during the first week post-irradiation, and thereafter LIs were similar to controls for the 5 months of the experiment. The LI after 10 and 12 Gy again showed a significant depression during the first week, but a significant increase followed (P = 0.01) in LI which peaked at 4 weeks after irradiation. LI returned to control values at 3-4 months and again rose significantly (P = 0.05) at 5 months. The first wave of proliferation corresponds to data showing an increase in surfactant in alveolar fluids within 2-6 weeks of 10-15 Gy of X rays; the second wave coincides with the pneumonitic phase, consistent with a delay before the alveolar epithelial continuity is sufficiently compromised by low rates of type I cell loss to trigger a compensatory wave of type II cell divisions. (author)