Clones, microcolonies or 'tumourlets' in irradiated lung

The formation of "tumourlets" in irradiated lung as described by Talerman (1973) is a good example of the tendency to form dysplastic nodules seen in many tissues regenerating after certain critically defined doses of radiation. These nodules probably represent discrete clones formed when the depleted tissues are repopulated by the progeny of single surviving cells or small clusters of clonogenic cells. If so, they are analogous to the splenic nodules formed by surviving "colony-forming units" after haemopoietic depletion (Till and McCulloch, 1961), to the pigment mosaics observed in irradiated human skin (Cohen, 1966), or to the microcolonies produced in the intestinal mucosa of mice following irradiation (Withers and Elkind, 1970).