Growth and survival of irradiated, cultured mouse thymic non-lymphoid cells

In vitro cultures of C57Bl/6J mouse thymic, non-lymphoid cells (TNL) showed characteristic exponential growth curves with control cultures reaching confluence in approximately 27 days. Depression of growth following gamma radiation was dose-dependent with no growth observed cultures irradiated with doses greater than 800 rads. TNLC single cell survival curves determined by electronic cell counting yielded a 2-component curve with a Do = 170 rads, n = 1.25 and Dq = 35 rads, which is compatible with the value expected for epithelial cells. However, morphologic observations suggested that the second component of the curve was due to a radioresistant cell population together with a 'background' of non-dividing, damaged or dying attached cells and associated debris which had been detected by the electronic cell counter. This portion of the curve had a Do of approximately 600 rads. Stripping this background from the initial curve resulted in a curve with a Do = 125 rads, n = 1.35 and Dq = 40 rads. These parameters are more characteristic of macrophages than epithelial cells. Subsequent cytochemical staining demonstrated that many, but not all cultured TNLC were macrophages.