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The dose survival and regeneration characteristics of irradiated mouse skin epithelial cells were studied using an in situ, in vivo cell cloning technique. The cells surviving a first dose of X-rays are characterized 24 hours later by a survival curve having the same slope as that for cells not previously irradiated. The value for (D₂-D₁) ₂₄ ₇ₑ was about 350 to 375 rads. The value of (D₂-D₁) increased by an average of about 100 rads per day with extension of fractionation intervals from 1 to 6 days. At intervals of less than 24 hours, a typical fluctuating pattern of recovery was observed. It is postulated that some repopulation by surviving cells occurs within 24 hours of a first dose of less than 800 rads and that this proceeds in the 5 days thereafter with a doubling time for the clonogenic population of approximately 22 hours. Evidence is presented for the differentiation of about 25% of each generation of cells during rapid re-epithelialization.
H. Rodney Withers (Sun,) studied this question.