3 AS OF REPOPULATION DURING FRACTIONATED-IRRADIATION OF SQUAMOUS EPITHELIA - ASYMMETRY LOSS, ACCELERATION OF STEM-CELL DIVISIONS AND ABORTIVE DIVISIONS

Purpose: To analyse the time-course and efficacy of repopulation in sq uamous epithelia, and to describe possible mechanisms of this regenera tion response. Material and methods: Experimental and clinical studies of repopulation in squamous epithelia have been reviewed to outline g eneral features of repopulation during fractionated radiotherapy. Resu lts: Repopulation processes result in a relative increase in re-irradi ation tolerance, which can be quantified as the dose equivalent compen sated. Conclusions: If the stem-cell concept is accepted, changes in r esidual tissue tolerance must be based on net production of stem cells . For this, normal asymmetrical stem-cell divisions that render equal numbers of stem cells and differentiating daughters, must turn into sy mmetrical divisions with the production of two stem cells. Furthermore , the rate of change in residual tolerance indicates that the stem-cel l proliferation rate is increased. In addition to these changes in the stem-cell proliferation pattern, a limited number of divisions by ste rilized cells contributes to overall cell production and maintenance o f tissue function. A valid model of repopulation in squamous epithelia hence must be based on three distinct mechanisms summarized as the th ree As: Asymmetry loss, Acceleration of stem-cell divisions, and Abort ive divisions.