The objective of this study was to determine the radiosensitivity to X-rays of spermatogonial stem cells in 3H1 (C3H/101 hybrid) mice during the cycle of the seminiferous epithelium. In particular whether or not, in this mouse, the radiosensitivity of the stem cells changes with the varying proliferative activity of the stem cell population during the epithelial cycle.
A dose response experiment was carried out in which 3H1 mice received graded doses of X-rays ranging from 0.5 to 10 Gy. In sections of the testes of these mice taken at day 10 after irradiation, the number of undifferentiated spermatogonia was counted in each epithelial stage as a measure of the number of surviving stem cells. This made it possible to derive D0 values for stem cell killing in the different stages of the spermatogenic cycle. The testis weights were also measured at ten days after irradiation. The very low dose of 0.5 Gy had very little influence on the testis weight. After 1 Gy of X-rays there was a marked drop in testis weight, which is most likely caused by the depletion of differentiating spermatogonia, which form the most radiosensitive cell population in the testis. Above 1 Gy there was a more or less gradual decrease in testis weight.
Undifferentiated spermatogonia from 4 animals (per dose) were counted for the doses 1.0, 2.0, and 3.0 Gy, and from 2 animals (per dose) for the doses 0.5 and 5.0 Gy, and the means of the numbers present at ten days after irradiation determined. The length of the spermatogenic cycle was assumed to be 207 hours, and the lengths of the stages was calculated from the frequency of appearance.
The data of the cell counts were placed on dose response graphs. From the curves a first indication of the radiosensitivity of the different stages of the spermatogenic cycle was obtained by means of an unweighed linear regression analysis. There does not seem to be an indication of a shoulder region in most of the response curves, except perhaps in those of th e stages mid/late I through late II.
The D0 values obtained from the dose response graphs were combined to give an indication of the radiosensitivity of the spermatogonial stem cells throughout the cycle of the seminiferous epithelium. The radiosensitivity seems to be highest when the spermatogonial stem cells are in a nonproliferating state (stage VIII). This has also been found for the CBA mouse after exposure to X- or neutrons. The highest radioresistance seems to occur in stages XII and V. However, it should be remembered that these D0 values are deduced from an unweighed regression analysis, and that the cell counts have not been completed yet. When the cell counts of the separate stages are taken together, an overall picture of the radiosensitivity of the total population of spermatogonial stem cells can be obtained. The dose response for the total population of stem cells indicates a radiosensitivity with a DO of 2.1 Gy X-rays, which is somewhat higher than the D0 of 1.8 Gy X-rays that has been found for the CBA mouse.