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 (D_o, 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 (D_o, 0.43 Gy) than the other phases of the cell cycle (D_o, 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.