Studies on the Cellular Basis of Radiation Lethality

Summary

A model is presented which defines the parameters of the curve for neoblast (stem cell) survival of reproductive integrity in terms of the parameters of the planarian radiation mortality curve. Two constants of the model, namely, the initial number of neoblasts (N) and the repopulation probability (α) for the independent repopulation of the irradiated planarian tissues by surviving neoblasts are defined and, for the species Dugesia lugubris, measured. The assumption that the repopulation probability for each neoblast is independent of the number of neoblasts is confirmed experimentally for diploid and triploid neoblasts of exogenous and endogenous origin. The repopulation probability for diploid (endogenous and exogenous) and triploid (endogenous only) neoblasts are not significantly different. Triploid exogenous neoblasts, however, are only 16 per cent as effective as endogenous triploid or exogenous diploid neoblasts. Values of D₀ and NαE (E = extrapolation number) are given for diploid, triploid and tetraploid neoblasts of D. lugubris, and for four other species.