The ultrastructure of chronic radiation damage in rat skin.

Chronic radiation skin ulcers were produced in rats with the administration of 5,000 or 8,000 rads. Electron microscopy revealed two phases of damage, the first from seven to 14 weeks and the second from 14 to 54 weeks. In the first phase, significant nuclear and cytoplasmic degeneration was prominent. During the second phase, lysosomes were common along with unusual cytoplasmic inclusions. Myofibroblasts were observed in the second phase only, suggesting that delayed would contraction in radiation ulcers was the result of the delayed onset of myofibroblast development.