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The Cell Cycle of Spermatogonial Colony Forming Stem Cells In the Cba Mouse After Neutron Irradiation

Abstract. In the CBA mouse testis about 10% of the stem cell population is highly resistant to neutron irradiation (Do, 0.75 Gy). Following a dose of 1.50 Gy these cells rapidly increase their sensitivity towards a second neutron dose and progress fairly synchronously through their first post‐irradiation cell cycle. From experiments in which neutron irradiation was combined with hydroxyurea it appeared that in this cycle the S‐phase is less radiosensitive (Do, 0.43 Gy) than the other phases of the cell cycle (Do, 0.25 Gy). From experiments in which hydroxyurea was injected twice after irradiation the speed of inflow of cells in S and the duration of S and the cell cycle could be calculated. Between 32 and 36 hr after irradiation cells start to enter the S‐phase at a speed of 30% of the population every 12 hr. At 60 hr 50% of the population has already passed the S‐phase while 30% is still in S. the data point to a cell cycle time of about 36 hr, while the S‐phase lasts 12 hr at the most.