Repair of radiation damage to capillary endothelium.

Two types of repair occurring in capillary endothelium after irradiation were investigated. The system used was the rat subcutis airpouch, in which the capillary endothelium was stimulated to proliferate by a combined mechanical and chemical stimulus. Assay was by an angiographic technique. The system allows temporal separation between the moment of irradiation and the moment of stimulation to proliferation. The results indicate that capillary endothelium shows the typical "Elkind-type" of time-recovery curve. When the stimulus to proliferation after a single dose of X rays is delayed, however, the D0 value rises from 168 rads (immediate stimulation) to 273 rads (stimulation to proliferation delayed for 32 days). The conclusion can therefore be made that many more endothelial cells survive in the tissues after radiotherapy than one would infer from the simple direct proliferation assays.