Repair and recovery in the epithelial and vascular connective tissues of pig skin after irradiation.

Using split-dose experiments, with varying time intervals between two equal fractions, the total repair capacity and the time of onset of additional recovery was determined for both early and late responses in pig skin. The early epidermal response was studied after beta-irradiation and 250 kV X-rays were used to investigate dermal changes. Based on the results of seven separate single dose studies the ED50 value ( +/- SE) for early moist desquamation was 27.76 +/- 0.91 Gy. With intervals of one and 14 days between two equal fractions similar ED50 values of 35 Gy were obtained. This suggested a recovered dose of approximately 7 Gy for epithelial desquamation, a repair capacity for sublethal injury of 20-25%. Additional recovery possibly due to repopulation, was observed with intervals of greater than or equal to 21 days between fractions. The rate of additional recovery was linearly related to the time interval between doses and was equivalent to 74 cGy/day. Recovery from the first dose was complete within 6 weeks. Evidence for radiation-induced tissue hypoxia was obtained when longer time intervals between doses were used. The more subjective early erythema reaction was also assessed. This reaction produced a similar estimate for the repair capacity and for the time of onset of recovery to that obtained using moist desquamation. This agreement was not maintained with intervals of greater than or equal to 28 days between doses due to an artefact associated with the way erythema reactions were assessed. After irradiation with single doses of X-rays ED50 values of 18.59 +/- 0.49 and 20.53 +/- 0.35 Gy were obtained for the dermal reactions of dusky/mauve erythema and necrosis, respectively. The recovered doses for the dermal responses, with intervals of 1 and 28 days between fractions, were similar, approximately 4.2 Gy, indicating a total repair capacity of 20-25%. Additional dermal recovery was seen only with intervals of greater than 28 days between doses. There was no evidence for "slow repair". Surprisingly complete recovery from the first dose was suggested with an interval of 16 weeks between doses. This finding might be influenced by radiation-induced hypoxia. The time of onset of additional repopulation/recovery and the latency for tissue impairment in epidermal and dermal tissues in pig skin were compared with those for other early and late responding tissues.