The radiosensitivity of microcolony- and macrocolony-forming cells in mouse tail epidermis.

A new microcolony technique is described for measuring the survival of colony-forming cells in mouse tail epidermis. The survival curve is characterised by D0 = 2.70 +/- 0.12 Gy. The number of microcolonies per cm2 is similar to the number of macrocolonies after high doses, which shows for the first time that all microcolonies (greater than or equal to 32 cells) in epidermis develop into macrocolonies. At low doses the number of macrocolonies underestimates the number of colony-forming cells because of coalescence of microcolonies to form macrocolonies. This results in a lower apparent sensitivity of macrocolony-forming cells by a factor of about 1.5. About 3% of basal cells in tail epidermis appear to be capable of colony formation.