Two-stage cell shrinkage and the OER for radiation-induced apoptosis of rat thymocytes.

Suspensions of rat thymocytes were given 0.09-100 Gy using 60Co gamma-rays. The radiation-induced changes in the thymocytes were examined from minutes to hours post-irradiation using electron microscopy, agarose gel electrophoresis, staining and Coulter Counter sizing. Sizing by Coulter Counter showed, for the first time, that thymocytes which undergo apoptosis shrink in two distinct stages, first by a sudden decrease from an original volume of 99 microns 3 to a volume of 76 microns 3, followed by a gradual decrease to 57 microns 3 over the space of a few hours. The oxygen enhancement ratio for apoptosis was measured to be about 3.5, similar to the value for reproductive death for many mammalian cells.