The Use of Elongate Microbeams of X-Rays for Simulating the Effects of Cosmic Rays on Tissues: A Study of Wound Healing and Hair Follicle Regeneration

In a comparative biological study, mice were irradiated with a microbeam of x rays, 150 microns wide, and with a control macrobeam, 5 mm in diameter. At 9 days after irradiation, the microlesions contained visibiy damaged hair follicles, which were neither hypertrophied nor hyperplastic. After 18 days of healing, the surviving hair follicles in the microlesions were thin, disorganized columns of epithelial cells. In contrast, many surviving hair follicles in the large irradiated areas were hypertrophied and hyperplastic, and they often contained keratogenous cysts. The divergent results suggest that the healing of microlesions is unusually rapid and differs qualitatively from that observed in large lesions. The possible reasons for the differences are discussed in relation to the problems of radiation damage caused by heavy cosmic-ray primaries, grid irradation, and interface effects at the borders of large irradiated areas. The results indicate that the entire healing process should be studied separately in large and small lesions so that qualitative as well as quantitative differences in injury and repair can be properly evaluated. (P.C.H.)