The ability of irradiated intermitotic cells of the rat lens epithelium to respond to a mitogenic stimulus produced by wounding was used to reveal latent radiation damage and to measure cellular recovery. Only the eyes were x-irradiated with 1000 rads. Whole mounts of lens epithelia,3 H- TdR cell labeling, and autoradiographs were utilized to evaluate the response. In unirradiated epithelia, a wave of DNA synthesis began 12-14 hr after wounding and a wave of mitosis began 10 hr later. Irradiation 1 hr before or after wounding delayed and suppressed the waves of DNA synthesis and mitosis. Some recovery occurred during a 1-month interval between irradiation and wounding; the cells responded with no delay, but the intensity of the response (i.e., the amplitude of the wave of cell labeling) was only about half that of control (unirradiated) epithelia. Irradiation at 12 hr after wounding, 2 hr before the onset of DNA synthesis, had little effect on DNA synthesis, but did delay and decrease the amplitude of the mitotic response. Irradiation at 22.5 hr after wounding, 1.5 hr before the onset of mitosis, delayed mitosis at least 7-8 hr. In unirradiated epithelia, the first of the responding intermitotic ( G0) cells made the transition from G0 to G1 and completed G1 activities (including preparations for DNA synthesis) in about 12 or 13 hr, began DNA synthesis (the S period) 13-14 hr after wounding, and began mitosis (the M period) about 24 hr after wounding, about 10 hr after they had begun DNA synthesis. Some of these cells, at least, may have completed the S period in 4-5 hr and G2 activities (including other preparations for mitosis) in about 6 more hr. We have postulated that the responding cells entered into and progressed through the cell cycle as successive groups or pulses of cells centered about 4 hr apart.

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