Comparison of the chromosomal radiosensitivity of blood lymphocytes and stem-cell spermatogonia in the rhesus monkey and the mouse

Dose-effect relationship for X-ray-induced translocations in spermatogonia of normal and T70H translocation heterozygous mice

The induction of reciprocal translocations in stem cell spermatogonia was studied in normal mice and T70H translocation heterozygotes using spermatocyte analysis many cell generations after irradiation. Dose-response relationships were constructed employing acute X-ray exposures of 1-10 Gy. The obtained results confirmed our earlier observations that T70H heterozygous mice were overall less sensitive to the induction of translocations compared to normals. The shape of the dose-response curve for T70H heterozygotes was also clearly different from normal mice. No simple correlations between cell killing, measured as testis weight loss, and observed frequencies of chromosomal anomalies could be established. In general, selective elimination of translocation carrying stem cells of T70H heterozygotes seem to be mainly responsible for the differences in induction pattern between the two types of mice.

The lymphocytes of small mammals. A model for research in cytogenetics?

The influence of nuclear characteristics such as chromosome number, chromosome arm number or amount of R-band material on the yield of radiation-induced chromosome aberrations has been re-evaluated on the basis of some recent experiments with the harlequin-staining method to distinguish the first from subsequent divisions. The results obtained up to now show that differences in cell kinetics of stimulated peripheral blood lymphocytes, causing a different distribution of first and following mitoses at the time of observation, can result in an underestimate of the incidence of unstable chromosome aberrations such as dicentric chromosomes when the observations are performed at a standardized fixation time without taking into account the cell generation. If, however, only first-division mitoses are considered, the radiosensitivity of the cells appears to be independent on nuclear characteristics. Furthermore, observations on whole-body irradiated animals show that, owing to the very short survival time of lymphocytes carrying chromosome aberrations, mammals generally available for such experiments represent inappropriate models when cytogenetic effects in somatic cells are to be followed for long periods of time after an acute exposure to ionizing radiation or after a chronic exposure.

Absence of correlation between the chromosomal radiosensitivity of peripheral blood lymphocytes and stem-cell spermatogonia in mammals

To evaluate the reliability of quantitative extrapolation of radiation-induced chromosomal damage from somatic cells to germ cells, data on the effects of several biological and physical factors on the chromosomal radiosensitivity of blood lymphocytes and stem-cell spermatogonia have been collected from the literature. The results show that most of the factors considered, such as chromosomal constitution, age, genetic constitution, species, sampling time and dose fractionation, had differential effects on the induction of chromosomal aberrations in both systems. These differential effects can easily be explained in terms of the biological differences between in-vitro-stimulated peripheral blood lymphocytes and stem-cell spermatogonia. It is concluded that only direct experiments on germ cells of higher primates and man can be used for a quantitative estimation of human genetic radiation risks arising from structural chromosomal aberrations.

Induction by X-rays of chromosomal aberrations in somatic and germ cells of mice with different karyotypes

Stem-cell spermatogonia of +/+, T70H translocation heterozygous and Ts(1(13))70H tertiary trisomic mice were irradiated with various doses of x-rays. Blood lymphocytes of +/+ and Ts(1(13))70H males were irradiated with 150 rad X-rays. Contrary to earlier findings (De Boer et al., 1977), tertiary trisomic mice with morphological abnormalities did not show an increased sensitivity for radiation-induced chromosome breaks. The yields of radiation-induced reciprocal translocations, scored in the primary spermatocytes, were equal for the +/+ and tertiary trisomic mice but were lower for the T70/+ males. Thus, no parallel could be established with the sensitivity of peripheral blood lymphocytes for the induction of dicentric chromosomes, where T70H/+ did not differ from +/+ mice (De Boer et al., 1977). Induced reciprocal translocations occurred randomly over late diplotene-early diakinesis and late diakinesis-metaphase I stages. Among the T70H/+ males (2 x 500 rad) there was no relation between the chiasma frequency per animal and the yield of induced translocations, though the translocation induction was positively correlated with the chiasma frequency on a per-cell basis. When all descendants of irradiated T70H spermatogonia were pooled, the induced translocation-originated multivalent contained more chiasmata in cells that had a higher chiasma score themselves. No relation could be found between the yield of induced translocations and the recovery of spermatogenesis after irradiation as expressed by the epididymal sperm count after 2 x 500 rad.

Clonal analysis of radiation-induced translocations in stem-cell spermatogonia of normal and T70H translocation heterozygous mice

7 T(1;13)70H/+ and 13+/+ male mice were given 2 doses of 250 rad acute X-rays separated by 24 h. The +/+ mice were analysed in 2 groups during the first meiotic division for induced translocations, on average 177 and 233 days after irradiation, and the T70H/+ mice were analysed in parallel with the second group of +/+ males. One testis was treated with normal air-drying procedures yielding a random sample of cells. The other testis was processed according to a new technique, which enable separate analysis of the various locations along the seminiferous epithelium where groups of cells are synchronously in the diakinesis-metaphase I stage of meiosis. The number of cells in such groups was estimated. Both capita epididymes were used for a sperm count. In agreement with an earlier finding, fewer induced translocations were recovered from the T70H/+ mice than from +/+ mice (10.6 versus 19.2%, air-drying technique). Estimates of the group sizes in combination with the occurrence of induced translocations yielded the following information. A synchronously moving group of diakinesis-metaphase I cells originates from, on average, 1.25 stem cells (Appendix). We found an indication for a reduction in group size by 33% when a clone originated from a stem cell carrying an induced translocation compared with a wild-type clone (see Appendix). Both, the data on group size and the sperm counts indicate that, 7 months after the irradiation, the seminiferous epithelium has not totally recovered. Final recovery seems to be slower or absent in the T70H/+ males. The data obtained from the T70H/+ heterozygotes indicate the stem-cell spermatogonia to be responsible for the reduction of the rate or translocation induction with this karyotype, either due to a reduced formation rate or due to a diminished capacity of some of the induced translocation-carrying stem cells to proliferate into a clone reaching the meiotic divisions.

Dose-response relationship for x-ray-induced reciprocal translocations in stem-cell spermatogonia of the rhesus monkey (Macaca mulatta)

The induction of reciprocal translocations in stem-cell spermatogonia of the rhesus monkey (Macaca mulatta) was studied after testicular X-irradiation of mature males (50, 100 and 200 rad) or whole-body irradiation of young males (200 and 300 rad). After the recovery of the germinal epithelium, cytogenetic analysis was carried out on spermatocytes descended from irradiated spermatogonia. Preparations of C-banded diakinesis-metaphase I were screened for translocation configurations. The frequencies of aberrations obtained were 0% at 0 rad, 0.36% at 50 rad, 0.86% at 100 rad, 0.99% at 200 rad and 0.68% at 300 rad, suggesting a humped dose-response relationship. There was no evidence for the contribution of a quadratic component to the yield in the lower dose range. A comparison of these results with those obtained for other mammals by a number of investigators shows that the frequencies of translocations in the rhesus monkey are much lower than those published for most other mammalian species.