Studies on chromosome aberrations in the eggs of mice fertilized in vitro after irradiation. II. Chromosome aberrations induced in mature oocytes and fertilized eggs at the pronuclear stage following X-irradiation

Effects of low-dose X-ray medical diagnostics on female gonads: Insights from large animal oocytes and human ovaries as complementary models

Diagnostic imaging has significantly grown over the last thirty years as indispensable support for diagnostic, prognostic, therapeutic and monitoring procedures of human diseases. This study explored the effects of low-dose X-ray medical diagnostics exposure on female fertility. To aim this, cumulus-oocyte complexes (COCs) recovered from the ovaries of juvenile sheep and human ovaries were used as complementary models for in vitro studies. In the sheep model, the effects of low-dose X-rays on oocyte viability and developmental competence were evaluated. In human ovaries originated from two age group (21–25 and 33–36 years old) subjects with gender dysphoria, X-rays effects on tissue morphology, follicular density and expression of apoptosis-related (NOXA, PUMA, Bcl2, Bak, γH2AX) and cell cycle-related genes (p21 and ki67) were investigated. It was noted that in sheep, the minimum dose of 10 mGy did not influence most of examined parameters at oocyte and embryo levels, whereas 50 and 100 mGy X-ray exposure reduced oocyte bioenergetic/oxidative activity but without any visible effects on oocyte and embryo development. In addition, blastocyst bioenergetic/oxidative status was reduced with all used doses. Overall data on human ovaries showed that low-dose X-rays, similarly as in sheep, did not alter any of examined parameters. However, in women belonging to the 33–36 year group, significantly reduced follicular density was observed after exposure to 50 and 100 mGy, and increased NOXA and Bax expression after exposure at 50 mGy. In conclusion, used low-doses of X-ray exposure, which resemble doses used in medical diagnostics, produce weak damaging effects on female fertility with increased susceptibility in advanced age.

Systematic review of experimental studies on the relative biological effectiveness of tritium

Tritium ((3)H) is a radioactive isotope of hydrogen. A number of factors combine to create a good deal of interest in tritium doses, both to workers and to members of the public. Tritium is ubiquitous in environmental and biological systems and is very mobile due to its occurrence as water. In this study we systematically review experimental data relating to tritium exposure with a view to assessing its low dose limiting relative biological effectiveness (RBE(max)). Interpretation of published experimental studies is complicated by the fact that the reference radiations varied, and doses and dose rates were frequently much higher than those normally received by humans. The four available animal carcinogenicity studies gave RBE values of about 2.5 with chronically-delivered gamma-ray reference, and about 1.2 with chronically-delivered X-ray reference. However, because of problems associated with the design and interpretation of the experiments, we do not consider that these RBE values should be taken to apply to the induction of cancer at low doses (i.e. they should not be interpreted as RBE(max)). Combining the six studies with chronic gamma-ray reference, with adequate quantitative data that examined endpoints apart from cell survival and related endpoints, yields an aggregate RBE estimate of 2.19 (95% CI 2.04, 2.33); the analogous combined RBE estimate using the three studies with chronic X-ray reference groups is 1.17 (95% CI 0.96, 1.39). Again, problems with the design, in particular the range of doses used in some of these studies, mean that these RBE values should also probably not be interpreted as RBE(max).

Spatial distribution of sperm-derived chromatin in zygotes determined by fluorescence in situ hybridization

Fluorescence in situ hybridization was used to determine the spatial distribution of chromatin in zygote pronuclei. A hybrid system involving golden hamster eggs and individual human sperm permitted use of DNA probes specific for the entire human chromosome 4, for the heterochromatic region on the long arm of the human Y chromosome and for unique DNA sequences on human chromosome 19. Chromosome 4 occupied a circumscribed domain in the pronuclei, similar to findings in somatic interphases. Unlike the situation in somatic interphases, the Y heterochromatin was extended throughout the first cell cycle. Pronuclear chromatin was extended 3- to 4-fold compared to somatic interphase chromatin. The extended pronuclear chromatin conformation is likely to affect a zygote's susceptibility to environmental hazards.

Changes in X-ray sensitivity of mouse eggs from fertilization to the early pronuclear stage, and their repair capacity

To study the changes in radiosensitivity of male and female genomes from fertilization to the pronuclear stage, the frequency of induced chromosome aberrations was examined at the first-cleavage metaphase in eggs fertilized with X-irradiated sperm, in eggs X-irradiated at the mature oocyte stage immediately before fertilization, and in fertilized eggs exposed to X-rays at various stages before DNA synthesis (1-5 h after insemination). Gametic treatment, fertilization and embryo culture were carried out in vitro. Most of the induced chromosome aberrations were chromosome-type aberrations, the frequency of chromosome fragments being the highest, followed by chromosome exchanges in both sperm and oocytes. The induction of chromosome-type aberrations was much higher in oocytes than in sperm. Chromosome-type aberrations were the main type also in fertilized eggs irradiated at the pre-DNA-synthetic stage. The radiosensitivity increased gradually with pronuclear formation (1-4h after insemination), but little difference in radiosensitivity was observed between eggs irradiated at 4 h and 5 h, corresponding to the stage when pronuclear formation was complete. The drastic change with pronuclear formation was found not only in radiosensitivity but also in the frequency of chromosome aberrations. The frequency of chromosome-type exchanges decreased drastically 2 h after insemination, and exchanges were barely observed at 5 h. The difference in radiosensitivity between male and female genomes also markedly changed with pronuclear formation, the chromosome aberration induction in the female genome being much higher than that in the male genome before accomplishment of pronuclear formation. The analysis of potentiation effects of 3-aminobenzamide and caffeine on the yield of X-ray-induced chromosome aberrations demonstrated that the increase of radiosensitivity and the decrease of chromosome-type exchange induction with pronuclear formation, may be closely correlated with alterations in chromatin configuration in the pronuclei and in repair capacity of fertilized eggs at the pre-DNA-synthetic stage. No evidence based on repair efficiency was found for the marked difference in radiosensitivity between male and female genomes during pronuclear formation.

Repair capacity of fertilized mouse eggs for X-ray damage induced in sperm and mature oocytes

To study the repair capacity of fertilized mouse eggs for X-ray damage induced in sperm and mature oocytes, the potentiating effects of 3 well-known repair inhibitors, arabinofuranosyl cytosine (ara-C), 3-aminobenzamide (3AB) and caffeine, on the frequency of induced chromosome aberrations were examined in eggs fertilized with X-irradiated sperm or in eggs irradiated with X-rays at the mature oocyte stage immediately before fertilization. Gametic treatment, fertilization and embryo culture were carried out in vitro. Ara-C treatment was done only in the pre-DNA replication period, while treatment with 3AB and caffeine was continuous from fertilization to the first-cleavage metaphase. The induction of chromosome aberrations by exposing sperm or oocytes to X-rays was remarkably potentiated by post-treatment incubation in the presence of each of the 3 inhibitors. This result indicates the possibility that X-ray damage induced in sperm or oocytes is reparable in the fertilized eggs and that various types of repair processes are involved.

The one-cell mouse embryo: cell cycle-dependent radiosensitivity and development of chromosomal anomalies in postradiation cell cycles

One-cell mouse embryos were irradiated with X-rays at different cell cycle stages. Examination of structural chromosomal anomalies and of micronucleus formation in postradiation mitoses and interphases demonstrated cell cycle-dependent radiosensitivities in the order: late G2 phase greater than G1 phase greater than S phase greater than early G2 phase greater than stage of decondensing nuclei. Comparison of the quality and quantity of chromosomal aberrations from the first to the third mitosis led to the conclusion that new chromosomal anomalies were formed in the course of postirradiation cell cycles. This hypothesis was supported by an increasing number of micronuclei from 24 to 48 h post-conception. In addition to structural chromosomal aberrations, radiation-induced chromosome loss was observed with a frequency that was obviously independent of the exposed cell cycle phase. Loss of acentric chromosome fragments and of single chromosomes contributed to the micronucleus formation.

Analysis of structural and numerical chromosomal anomalies at the first, second, and third mitosis after irradiation of one-cell mouse embryos with X-rays or neutrons

One-cell mouse embryos were irradiated with X-rays or neutrons. Analysis of the first, second, and third postradiation mitoses revealed that the yields of structural aberrations increased linearly after exposure to both radiation qualities. For X-rays the aberration frequency decreased from the first to the third mitosis, whereas after neutrons it decreased from the first to the second mitosis but then increased in the third mitosis. RBEs of 4.7, 4.8, and 7.4 were calculated for the corresponding mitoses. It was clearly demonstrated that new aberrations were produced after the first postradiation mitosis and expressed during the second and third mitosis. Chromosome loss also increased with increasing radiation dose at the second mitosis. An RBE of 2 was calculated for this effect. Comparing the presented data with previous investigations on embryonic and fetal death after prenatal irradiation, it was concluded that the high radiosensitivity of the one-cell embryo is due to the induction of structural as well as of numerical chromosome aberrations.

Chromosomal analysis in mouse eggs fertilized in vitro with sperm exposed to ultraviolet light (UV) and methyl and ethyl methanesulfonate (MMS and EMS)

Chromosome aberrations were analyzed at the first-cleavage metaphase of mouse eggs fertilized in vitro with sperm exposed to ultraviolet light (UV) as well as to methyl and ethyl methanesulfonate (MMS and EMS). The frequencies of chromosome aberrations markedly increased with dose of UV as well as with concentration of MMS and EMS. In the UV-irradiation group, the frequency of chromosome-type aberrations was much higher than that of chromatid-type aberrations. About 90% of chromosome aberrations observed in the eggs following MMS and EMS treatment to sperm were chromosome type in which the frequency of chromosome fragments was the highest. The effects of UV on the induction of chromosome aberrations were clearly potentiated by post-treatment incubation of fertilized eggs in the presence of Ara-C or caffeine, but the effects of MMS and EMS were not pronounced by post-treatment of Ara-C or caffeine. The results indicate a possibility that UV damage induced in mouse sperm DNA is reparable in the eggs during the period between the entry of sperm into the egg cytoplasm and the first-cleavage metaphase.

Chromosome aberrations induced by tritiated water or 60Co gamma-rays at early pronuclear stage in mouse eggs

The induction of chromosome aberrations in mouse eggs by exposure to HTO beta-particles and 60Co gamma-rays at the early pronuclear stage was examined at the first-cleavage metaphase by using an in vitro fertilization technique. Eggs at the pronuclear stage were exposed to beta-particles in a chemically defined medium containing tritiated water (HTO) for 2 h at 3-5 h after insemination. Other eggs at the same stage were exposed to gamma-rays from 60Co during the same period. The dose-response relationships for frequencies of chromosome aberrations per egg were fitted to a linear-quadratic model for HTO beta-particles, and to a linear model for 60Co gamma-rays. The chromosome aberrations were mainly chromosome-type, and the majority of all aberrations were fragments. RBE values of HTO beta-particles relative to 60Co gamma-rays and acute X-rays, which were estimated from the ratio of the linear regression coefficients over 0.05-Gy range, were 2.0 and 1.6, respectively.