Exposure of female mice to ethylene oxide within hours after mating leads to fetal malformation and death

Protection of the gametes embryo/fetus from prenatal radiation exposure

There is no convincing evidence of germline mutation manifest as heritable disease in the offspring of humans attributable to ionizing radiation, yet radiation clearly induces mutations in microbes and somatic cells of rodents and humans. Doses to the embryo estimated to be in the range of 0.15-0.2 Gy during the pre-implantation and pre-somite stages may increase the risk of embryonic loss. However, an increased risk of congenital malformations or growth retardation has not been observed in the surviving embryos. These results are primarily derived from mammalian animal studies and are referred to as the "all-or-none phenomenon." The tissue reaction effects of ionizing radiation (previously referred to as deterministic effects) are congenital malformations, mental retardation, decreased intelligence quotient, microcephaly, neurobehavioral effects, convulsive disorders, growth retardation (height and weight), and embryonic and fetal death (miscarriage, stillbirth). All these effects are consistent with having a threshold dose below which there is no increased risk. The risk of cancer in offspring that have been exposed to diagnostic x-ray procedures while in utero has been debated for 55 y. High doses to the embryo or fetus (e.g., >0.5 Gy) increase the risk of cancer. Most pregnant women exposed to x-ray procedures and other forms of ionizing radiation today received doses to the embryo or fetus <0.1 Gy. The risk of cancer in offspring exposed in utero at exposures <0.1 Gy is controversial and has not been fully resolved. Diagnostic imaging procedures using ionizing radiation that are clinically indicated for the pregnant patient and her fetus should be performed because the clinical benefits outweigh the potential oncogenic risks.

Differential susceptibility of rat embryos to methyl methanesulfonate during the pregastrulation period

The effects of exposure of pregnant rats to methyl methanesulfonate (MMS), an alkylating agent, during the pregastrulation period on embryonic and placental development were investigated. SD rats were treated orally with a single dose of MMS (200 mg/kg) in the morning of gestation days 0, 1, 2, 3, 4, 5, or 6 (GD0 to GD6 groups, respectively). The uterine contents including fetuses and placentas of the dams were examined on gestation day 20. The individual fetuses and placentas were weighed, and the fetuses were examined for external, visceral and skeletal anomalies. The progress of ossification was also evaluated. Both pre- and postimplantation embryonic mortalities were higher in the GD0 group than in the control group. The postimplantation loss was also increased for the GD3, GD4 and GD6 groups. Fetal malformations were rare in survivors of all the MMS-treated groups. Intrauterine growth retardation was apparent for fetuses in groups GD5 and GD6. In addition, placental weight was reduced in the GD6 group, but it was increased in the GD0 group. Effects of MMS on embryonic mortality or on fetal or placental growth were absent or minimal in the GD1 and GD2 groups. These results suggest that the susceptibility of rat embryos to MMS varies during the pregastrulation period.

Gender differences in the induction of chromosomal aberrations and gene mutations in rodent germ cells

Germ cell mutagenicity testing provides experimental data to quantify genetic risk for exposed human populations. The majority of tests are performed with exposure of males, and female data are relatively rare. The reason for this paucity lies in the differences between male and female germ cell biology. Male germ cells are produced throughout reproductive life and all developmental stages can be ascertained by appropriate breeding schemes. In contrast, the female germ cell pool is limited, meiosis begins during embryogenesis and oocytes are arrested over long periods of time until maturation processes start for small numbers of oocytes during the oestrus cycle in mature females. The literature data are reviewed to point out possible gender differences of germ cells to exogenous agents such as chemicals or ionizing radiation. From the limited information, it can be concluded that male germ cells are more sensitive than female germ cells to the induction of chromosomal aberrations and gene mutations. However, exceptions are described which shed doubt on the extrapolation of experimental data from male rodents to the genetic risk of the human population. Furthermore, the female genome may be more sensitive to mutation induction during peri-conceptional stages compared to the male genome of the zygote. With few exceptions, germ cell experiments have been carried out under high acute exposure to optimize the effects and to compensate for the limited sample size in animal experiments. Human exposure to environmental agents, on the other hand, is usually chronic and involves low doses. Under these conditions, gender differences may become apparent that have not been studied so far. Additionally, data are reviewed that suggest a false impression of safety when responses are negative under high acute exposure of male rodents while a mutational response is induced by low chronic exposure. The classical (morphological) germ cell mutation tests are not performed anymore because they are animal and time consuming. Nevertheless, information is needed to place genetic risk extrapolations on more solid grounds and thereby to prevent an increased genetic burden to future generations. It is pointed out that modern molecular methodologies are available now to experimentally address the open questions.

Increased Risks of Term Low-Birth-Weight Infants in a Petrochemical Industrial City with High Air Pollution Levels

This study investigated the influence of petrochemical air pollution on birth weight. Birth data on 92,288 singleton infants with gestational periods of 37-44 wk born in a petrochemical industrial city (Kaohsiung, n = 31,530) with severe pollution or a nonpetrochemical industrial city (Taipei, n = 60,758) in Taiwan between 1995 and 1997 were included in this analysis. Air pollutant concentration derived from routinely monitored data showed significantly higher concentrations of SO2, O3, and PM10 in Kaohsiung. Infants with low birth weight (LBW) were significantly more prevalent in Kaohsiung (2.4%) than in Taipei (2.1%). Multivariable logistic regression analysis with adjustment for gestation age, gender, birth order, season of birth, maternal age, and maternal education further suggested that LBW risk in Kaohsiung was 13% higher than that in Taipei (odds ratio [OR] = 1.13, 95%, confidence interval [CI] = 1.03-1.24).

Association between maternal exposure to elevated ambient sulfur dioxide during pregnancy and term low birth weight

This retrospective cohort study investigated whether the risk of delivering full term (37-44 completed weeks of gestation) low birth weight (LBW) infants is associated with differences in exposure to air pollutants in different trimesters. Full-term infants (37 completed weeks of gestation) with a birth weight below 2500 g were classified as term LBW infants. The study infants comprised 92,288 full-term live singletons identified from the Taiwan birth registry and born in the city of Taipei or Kaoshiung in Taiwan between 1995 and 1997. Maternal exposures to various air pollutants including CO, SO2, O3, NO2, and PM10 in each trimester of pregnancy was estimated as the arithmetic means of all daily measurements taken by the air quality monitoring station nearest to the district of residence of the mother at birth. The multivariable logistic regression model with adjustment for potential confounders was used to assess the independent effect of specific air pollutants on the risk of term LBW. This study suggested a 26% increase in term LBW risk given maternal ambient exposure to SO2 concentration exceeding 11.4 ppb during pregnancy compared to low exposure (<7.1 ppb) (OR=1.26, 95% CI=1.04-1.53). Since the relative risk of term LBW was reassessed according to exposure level in each trimester, mothers exposed to >12.4 ppb of SO2 in the last trimester showed 20% higher risk (OR=1.20, 95% CI=1.01-1.41) of term LBW delivery than mothers with lower exposure (<6.8 ppb). No significant elevation ORs was observed for other air pollutants.

TRAIL and KILLER Are Expressed and Induce Apoptosis in the Murine Preimplantation Embryo1

TRAIL (tumor necrosis factor [TNF]-related apoptosis-inducing ligand) and KILLER are a death-inducing ligand and receptor pair that belong to the TNF and TNF-receptor superfamilies, respectively. To date, only one apoptosis-inducing TRAIL receptor (murine KILLER [MK]) has been identified in mice, and it is a homologue of human Death Receptor 5. Whereas the expression of other death receptors, such as Fas and TNF receptor 1 have been documented in mammalian preimplantation embryos, no evidence currently demonstrates either the presence or the function of TRAIL and its corresponding death receptor, MK. Using reverse transcription-polymerase chain reaction and confocal immunofluorescent microscopy, we found that both TRAIL and MK are expressed from the 1-cell through the blastocyst stage of murine preimplantation embryo development. These proteins are localized mainly at the cell surface from the 1-cell through the morula stage. At the blastocyst stage, both TRAIL and MK exhibit an apical staining pattern in the trophectoderm cells. Finally, using the TUNEL assay, we demonstrated that MK induces apoptosis in blastocysts sensitized to TRAIL via actinomycin D. Taken together, these data are the first to demonstrate the presence and function of TRAIL and MK, a death-inducing ligand and its receptor, in mammalian preimplantation embryos.

Joint models for toxicology studies with dose-dependent number of implantations

Many chemicals interfere with the natural reproductive processes in mammals. The chemicals may prevent the fertilization of an egg or keep a zygote from implanting in the uterine wall. For this reason, toxicology studies with pre-implantation exposure often exhibit a dose-related trend in the number of observed implantations per litter. Standard methods for analyzing developmental toxicology studies are conditioned on the number of implantations in the litter and therefore cannot estimate this effect of the chemical on the reproductive process. This article presents a joint modeling approach to estimating risk in toxicology studies with pre-implantation exposure. In the joint modeling approach, both the number of implanted fetuses and the outcome of each implanted fetus is modeled. Using this approach we show how to estimate the overall risk of a chemical that incorporates the risk of lost implantation due to pre-implantation exposure. Our approach has several distinct advantages over previous methods: (1) it is based on fitting a model for the observed data and, therefore, diagnostics of model fit and selection apply; (2) all assumptions are explicitly stated; and (3) it can be fit using standard software packages We illustrate our approach by analyzing a dominant lethal assay data set (Luning et al., 1966, Mutation Research, 3, 444-451) and compare ourresults with those of Rai and Van Ryzin (1985, Biometrics, 41,1-9) and Dunson (1998, Biometrics, 54, 558-569). In a simulation study, our approach has smaller bias and variance than the multiple imputation procedure of Dunson.

Embryo toxicity and teratogenicity of formaldehyde

C-14 formaldehyde crosses the placenta and enters fetal tissues. The incorporated radioactivity is higher in fetal organs (i.e., brain and liver) than in maternal tissues. The incorporation mechanism has not been studied fully, but formaldehyde enters the single-carbon cycle and is incorporated as a methyl group into nucleic acids and proteins. Also, formaldehyde reacts chemically with organic compounds (e.g., deoxyribonucleic acid, nucleosides, nucleotides, proteins, amino acids) by addition and condensation reactions, thus forming adducts and deoxyribonucleic acid-protein crosslinks. The following questions must be addressed: What adducts (e.g., N-methyl amino acids) are formed in the blood following formaldehyde inhalation? What role do N-methyl-amino adducts play in alkylation of nuclear and mitochondrial deoxyribonucleic acid, as well as mitochondrial peroxidation? The fact that the free formaldehyde pool in blood is not affected following exposure to the chemical does not mean that formaldehyde is not involved in altering cell and deoxyribonucleic acid characteristics beyond the nasal cavity. The teratogenic effect of formaldehyde in the English literature has been sought, beginning on the 6th day of pregnancy (i.e., rodents) (Saillenfait AM, et al. Food Chem Toxicol 1989, pp 545-48; Martin WJ. Reprod Toxicol 1990, pp 237-39; Ulsamer AG, et al. Hazard Assessment of Chemicals; Academic Press, 1984, pp 337-400; and U.S. Department of Health and Human Services. Toxicological Profile of Formaldehyde; ATSDR, 1999 [references 1-4, respectively, herein]). The exposure regimen is critical and may account for the differences in outcomes. Pregnant rats were exposed (a) prior to mating, (b) during mating, (c) or during the entire gestation period. These regimens (a) increased embryo mortality; (b) increased fetal anomalies (i.e., cryptochordism and aberrant ossification centers); (c) decreased concentrations of ascorbic acid; and (d) caused abnormalities in enzymes of mitochondria, lysosomes, and the endoplasmic reticulum. The alterations in enzymatic activity persisted 4 mo following birth. In addition, formaldehyde caused metabolic acidosis, which was augmented by iron deficiency. Furthermore, newborns exposed to formaldehyde in utero had abnormal performances in open-field tests. Disparities in teratogenic effects of toxic chemicals are not unusual. For example, chlorpyrifos has not produced teratogenic effects in rats when mothers are exposed on days 6-15 (Katakura Y, et al. Br J Ind Med 1993, pp 176-82 [reference 5 herein]) of gestation (Breslin WJ, et al. Fund Appl Toxicol 1996, pp 119-30; and Hanley TR, et al. Toxicol Sci 2000, pp 100-08 [references 6 and 7, respectively, herein]). However, either changing the endpoints for measurement or exposing neonates during periods of neurogenesis (days 1-14 following birth) and during subsequent developmental periods produced adverse effects. These effects included neuroapoptosis, decreased deoxyribonucleic acid and ribonucleic acid synthesis, abnormalities in adenylyl cyclase cascade, and neurobehavioral effects (Johnson DE, et al. Brain Res Bull 1998, pp 143-47; Lassiter TL, et al. Toxicol Sci 1999, pp 92-100; Chakraborti TK, et al. Pharmacol Biochem Behav 1993, pp 219-24; Whitney KD, et al. Toxicol Appl Pharm 1995, pp 53-62; Chanda SM, et al. Pharmacol Biochem Behav 1996, pp 771-76; Dam K, et al. Devel Brain Res 1998, pp 39-45; Campbell CG, et al. Brain Res Bull 1997, pp 179-89; and Xong X, et al. Toxicol Appl Pharm 1997, pp 158-74 [references 8-15, respectively, herein]). Furthermore, the terata caused by thalidomide is a graphic human example in which the animal model and timing of exposure were key factors (Parman T, et al. Natl Med 1999, pp 582-85; and Brenner CA, et al. Mol Human Repro 1998, pp 887-92 [references 16 and 17, respectively, herein]). Thus, it appears that more sensitive endpoints (e.g., enzyme activity, generation of reactive oxygen species, timing of exposure) for the measurement of toxic effects of environmental agents on embryos, fetuses, and neonates are more coherent than are gross terata observations. The perinatal period from the end of organogenesis to the end of the neonatal period in humans approximates the 28th day of gestation to 4 wk postpartum. Therefore, researchers must investigate similar stages of development (e.g., neurogenesis occurs in the 3rd trimester in humans and neonatal days occur during days 1-14 in rats and mice, whereas guinea pigs behave more like humans). Finally, screening for teratogenic events should also include exposure of females before mating or shortly following mating. Such a regimen is fruitful inasmuch as environmental agents cause adverse effec

Hyperglycemia and apoptosis: mechanisms for congenital malformations and pregnancy loss in diabetic women

Congenital malformations are the leading cause of perinatal death among infants of diabetic women. Abnormal fuel metabolism and hyperglycemia have been shown to disturb embryogenesis during the earliest pre- and postimplantation stages in mice. This review presents a new model to explain, in part, adverse pregnancy outcomes associated with diabetes. In this model, by altering gene expression in developing tissues, raised glucose concentrations led to premature programmed cell death in key progenitor cells of the mouse blastocyst or in emerging organ structures in the mouse postimplantation embryo, resulting in abnormal morphogenesis or miscarriage. Although recent studies are still somewhat speculative and have currently only been explored in the mouse, this paradigm is supported by examples in other cell systems, which include human-derived cell lines, thereby suggesting that these findings are also applicable to human pregnancy.

Analysis of cytogenetic and developmental effects on pre-implantation, mid-gestation and near-term mouse embryos after treatment with trichlorfon during zygote stage

Trichlorfon has been widely used in agriculture as a broad spectrum insecticide. We examined cytogenetic and developmental effects on early mouse zygotes exposed to trichlorfon in vivo. Pregnant female mice were intraperitoneally administered a single dose of trichlorfon (100 or 200mg/kg) at 6h post presumed conception and either sacrificed on day of gestation (dg) 3, 9 or 17 to assess the developmental toxicity and mutagenic effects on embryos. Mean cell number (dg 3) and somite number (dg 9) of embryos in the two trichlorfon-treated groups were significantly fewer than in the control group and the mean micronucleus (MN) number (dg 3) and the frequency of mosaic aneuploidies including monosomic or trisomic cell lines (dg 9) was significantly increased in both trichlorfon-treated groups compared with the control group. However, there was no difference in fetal body weight (dg 17) between the control and trichlorfon-treated groups and no increased incidence of external malformations was observed in the trichlorfon-treated groups. These findings suggest that acute exposure of trichlorfon around fertilization induces a high frequency of MN, mosaic aneuploidies and developmental retardation in pre-implantation and mid-gestation embryos, and thereafter these embryos with MN or chromosome damage appear to develop past mid-gestation and catch up with normal embryos by near-term.

Possible mechanism of congenital malformations induced by exposure of mouse preimplantation embryos to mitomycin C

ICR mice were treated intraperitoneally with mitomycin C at 5 mg/kg on day 3 of gestation. On day 18 of gestation, fetuses of treated dams were inspected for external, skeletal and visceral malformations. At 6 or 12 hr after mitomycin C treatment, the blastocysts were obtained from the uteri of treated dams and the degenerated cells within inner cell mass (ICM) and trophectoderm (TE) tissues were examined microscopically. On day 5, 8, 11, or 18 of gestation, the uteri of treated dams were obtained and those including embryos/fetuses and placentae were examined histologically. Finally, on each of gestational days 5-14, the blood of the treated dams was collected and the hematological parameters determined. Pre- and postimplantation losses in the dams treated with mitomycin C were significantly increased; increased frequency of abdominal wall defects and lumbar ribs in term fetuses, decreased fetal weight, and increased placental weight were noted as well. No significant increase in visceral malformations was found in term fetuses treated with mitomycin C. Frequency of degenerated cells within ICM and TE of blastocysts from dams treated with mitomycin C was significantly increased as compared with the controls. In dams treated with mitomycin C, decidua developed insufficiently and the trophoblast giant cell layer was not separated from the uterine lumen by maternal components; hemorrhage from the denuded trophoblast giant cell layer into the uterine lumen was noted. The number of erythrocytes, as well as hemoglobin concentration, hematocrit, and the percentage of reticulocytes in blood of dams treated with mitomycin C were significantly lower from days 6-12 of gestation, as compared with controls. The results of the present study showed that an increase in number of degenerated cells within blastocysts results in preimplantation loss and both maternal and embryonic hypoxia during major organogenesis results in postimplantation loss and congenital fetal malformations.

Impact of air pollution on reproductive health

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Paternal exposure of rabbits to lead: behavioral deficits in offspring

Paternal exposures to exogenous agents have been reported to produce a variety of developmental defects in the offspring. In experimental animals, these effects include decreased litter size and weight, increased stillbirth and neonatal death, birth defects, tumors, and functional/behavioral abnormalities-some of these effects being transmitted to the second and third generations. The majority of experimental studies assessing nervous system function of offspring following paternal exposures have utilized rats as the experimental animal, but other species can be used. The National Toxicology Program (NTP) has initiated studies to validate the rabbit as an animal model for human reproductive toxicity, because rabbits are the smallest laboratory animal from which ejaculates can be collected repeatedly. An important part of reproductive toxicology is assessment of the reproductive ability of males following exposure, as well as developmental and functional assessment of their offspring. This article describes a pilot study and a main study to investigate the feasibility of using rabbits to assess the functional effects of paternal exposure to lead. The pilot study included seven male rabbits per group exposed for 15 weeks to lead acetate sufficient to produce 0, 50, or 110 micrograms/dl blood lead. The main study included 15 male rabbits per group exposed for 15 weeks to lead acetate to produce 0, 20, 40, and 80 micrograms/dl blood lead. At the conclusion of the exposure, male rabbits were mated with unexposed females. These females carried their litters to term, delivered, and reared their own offspring. The offspring were weighed at 5, 10, 15, 20, 25, 30, and some at 35 days of age. They were also tested for exploratory activity in a standard figure-eight "maze" for 30 min/day on days 15, 20, 25, and 30. A second assessment of exploratory behavior, along with a simple test of aversive conditioning, was attempted in the pilot study, but was judged not to be suitable for the main study. Of the 21 male rabbits that were mated in the pilot study, 16 produced viable litters (6/7, 6/7, and 4/7 in control, low- and high-lead groups, respectively), with a mean number of 6 live births/litter in each treatment group (range 2-8). Of the 60 rabbits mated in the main study, 57 produced litters, and two rabbits died giving birth. Significant postnatal deaths were observed in all groups, with about one half of the offspring dying before testing was initiated at day 15. There were no treatment-related effects on offspring weight gain through wearing. The data suggest that paternal lead exposure of rabbits may reduce figure-eight activity on day 25, the time of peak activity in the offspring.

Monitoring of early human fetal development in women exposed to large doses of chemicals

The toxicological in-patient hospital in Budapest is responsible for the care of chemically poisoned persons from a population of 3 million. A population-based prospective epidemiological study of all pregnant women admitted from 1985 to 1993 was used to evaluate effects of large doses of chemicals on human fetal development. Of 559 self-poisoned pregnant women identified, two died from the poisoning. A total of 213 fetuses were in the first month of their postconception development. Of these, 126 had evaluated pregnancy outcomes: 111 ended in very early loss, 3 ended in clinical miscarriage, and 12 survived to delivery. (In addition 73 pregnancies were terminated and one pregnant woman died.) The 12 liveborn infants had two congenital abnormalities that were probably not related to their mother's self-poisoning. Though based on small numbers, these findings are consistent with an "all-or-nothing" effect of chemical poisoning very early in human gestation.

A genetic characterization of differences in the sensitivity to radiation-induced malformation frequencies in the mouse strains Heiligenberger, C57BI, and Heiligenberger x C57BI

We studied the frequency of malformations induced in two mouse strains (Heiligenberger, C57B1/6J) by exposure to x-rays 3 h after conception. Whereas there was a high number of malformed fetuses in Heiligenberger mice (mostly gastroschises) on day 19 of pregnancy, C57B1 did not respond to radiation exposure shortly after conception with an increased frequency of malformed fetuses. Cross-breeding of both strains revealed that no statistically significant increase in radiation-induced malformations was obtained in the F1 fetuses when the father was Heiligenberger and the mother C57B1. In the opposite case (Heiligenberger mother, C57B1 father) a small but statistically significant increase was observed.

Exposure to ethylene oxide during the early zygotic period induces skeletal anomalies in mouse fetuses

Exposure of mouse zygotes to ethylene oxide (EtO) has been shown to increase the incidence of external malformations among late fetuses [Generoso et al. (1987) Mutat. Res., 176:267-274; Rutledge and Generoso, (1989) Teratology, 39:563-572]. The present study was designed to determine whether EtO also affects the skeletal system. We report here the effects of varying times of exposure during the zygotic period on skeletal development. Female hybrid mice were injected intraperitoneally (IP) with 125 mg/kg EtO at 1, 3, 5, or 7 hr postmating. A positive control group consisted of female mice that were injected IP with 150 mg/kg EtO once daily between the 6th to 8th days of gestation. Day 17 fetuses were double-stained for "blind" examination of skeletal deviations and degree of ossification. Zygotic exposure to EtO significantly increased loss of conceptuses as well as the incidence of external defects, skeletal anomalies, and retarded ossification in live day 17 fetuses. An increase in the number of exposed fetuses with cleft sternum was observed with the highest rate (58.5%) occurring in fetuses whose mothers were exposed to EtO 3 hours postmating. Cleft sternum was seen in only 5% of fetuses exposed during the period of organogenesis and less than 1% of control fetuses. It is concluded that zygotic exposure to EtO produces a pattern of skeletal defects that differs from those observed following treatment with EtO during organogenesis.

Embryonic death, dwarfism and fetal malformations after irradiation of embryos at the zygote stage: studies on two mouse strains

Female mice of the BALB/c and CF1 strains were mated and irradiated with various doses of X-rays 7 h after presumed fertilization. 18 days later, females were killed and their uteri examined for prenatal mortality at the different stages of development. Living fetuses were weighed and examined for the presence of external malformations. A number of them were also examined for skeletal anomalies. Radiation induced mainly a dose-dependent increase of the preimplantation loss in the BALB/c strain and of the early postimplantation loss in the CF1 strain. Embryos of the BALB/c strain were refractory to the induction of teratogenic effects after such preimplantation irradiation. In CF1 mice, the frequency of malformed fetuses increased regularly after irradiation, the difference with controls being significant for the doses of 10, 50 and 100 cGy. Dwarfism occurrence also appeared to be increased by irradiation in this strain, although the importance of this effect varied depending on the criterion chosen for the assessment of dwarfs. With the definition proposed in the present paper, the increase in the frequency of dwarfs paralleled that of malformed fetuses, being significant after doses of 50 and 100 cGy. Irradiation did not increase the frequency of skeletal anomalies. A careful examination of the various data obtained to data led us to conclude that radiation may possibly be teratogenic in several mouse strains, when administered as early as during the one-cell stage and, to a lesser extent, during the following preimplantation stages. However, early prenatal mortality will remain by far the greatest risk associated with an exposure to radiation during this period. Moreover, the relativity of the risk of abnormality due to such irradiation should be considered in the context of the high prevalence of developmental defects spontaneously occurring during human pregnancy.

Ethylene oxide: evaluation of genotoxicity data and an exploratory assessment of genetic risk

A risk estimate of the heritable effects of ethylene oxide exposure, using the parallelogram approach, as suggested by Frits Sobels, is described. The approach is based on available data on the ethylene oxide-induced responses for the same genetic endpoint in somatic cells of both laboratory animals and humans, and for germ cell mutations in the same laboratory animal. Human germ cell effects are estimated. The available data sets for this approach were evaluated. We consider this as complementary to the genetic risk assessment carried out by U.S. EPA scientists, in which the risk from heritable (reciprocal) translocations induced by ethylene oxide was estimated. In the present study we restricted our assessment to dominant mutations. The sensitivity factor relating mouse to man was based on ethylene oxide-induced HPRT mutant frequencies in lymphocytes in vivo. From this comparison, it could be concluded that occupational exposure for 1 year to 1 ppm ethylene oxide would lead to a risk of a dominantly inherited disease in the offspring of 4 x 10(-4) above the background level. The uncertainty interval of this figure is quite large (0.6-28) x 10(-4). The values are compatible with the existing estimates of the corresponding risk from exposure to low LET radiation when the genotoxic potency ratio of ethylene oxide and radiation is considered. This risk estimation approach has allowed us to identify additional data that are required for a more complete risk estimation of the heritable effects of ethylene oxide, or indeed any mutagenic chemical.

Study of developmental abnormalities and deaths after human zygote exposure

Experimental studies indicated that the mouse zygote is susceptible to experimental induction of developmental anomalies including defects (mainly hydrops), growth retardation and mid- and late-gestational death with certain mutagenic agents. The material of the Hungarian Optimal Family Planning Program is appropriate to check this finding in a human material because participants were asked to visit the coworkers of the Program immediately after the first missed menstrual period and data concerning potentially hazardous environmental factors were obtained. At that time participants were immediately after the pre- and implantation period and they had no knowledge about their pregnancy outcomes. In 1994 the data of their pregnancy outcomes are available. Of 5453 evaluated pregnancies, 1167 were selected for this study because they visited the coworkers of the Program within 28 days post conception. Of 1167 pregnancies, 316 (27%) were exposed to some environmental factors, mainly drugs. A mild intrauterine growth retardation was found in the exposed group. The rate of congenital abnormalities and infant death did not differ between the exposed and unexposed groups. The detailed analysis of different congenital abnormality groups also did not show any significant difference between the exposed and unexposed groups. These negative results are explained by the fact that the observed environmental factors are not mutagenic, at least not in the dosage which was used, or the human zygote is not sensitive to mutagenic agents in the post-conceptional days.

Protein patterns in tissues of fetuses with radiation-induced gastroschisis

We have used two-dimensional gel electrophoresis coupled with computer-assisted data analysis to monitor protein expression in the liver of mouse fetuses with and without gastroschisis after X-irradiation of embryos during the 1-cell stage. A significantly higher frequency of changes in protein expression was observed in liver from irradiated fetuses with gastroschisis than from irradiated fetuses without gastroschisis. It was found that the frequency of abnormal protein patterns in the malformed fetuses is higher by approximately a factor of 2. Two proteins showed changes simultaneously in liver, kidney and/or skin of one individual fetus. The changes in protein expression probably result from mutations induced by the radiation exposure of the embryos at the 1-cell stage of prenatal development. We discuss these results in terms of increased mutation frequencies in irradiated fetuses with gastroschisis.

Fertility, reproduction, and genetic disease: studies on the mutagenic effects of environmental agents on mammalian germ cells

Because genetically based diseases have a major impact on human health, the National Institute of Environmental Health Sciences (NIEHS) has conducted a research and testing program for more than a decade to address chemical induction of heritable genetic damage in the germ cells of mammals. Although most genetic disease results from preexisting mutations, a portion is due to the occurrence of new mutations. The supposition that exposure to mutagenic chemicals contributes to the occurrence of new mutations in the human population is strongly supported by the results from animal models. Such studies clearly demonstrate the potential of environmental chemicals to induce mutations in both somatic and reproductive cells of mammals. This NIEHS program has become a leader in the identification of genetic hazards in the environment and in the acquisition of animal model data used by regulatory agencies in assessing genetic risks to human health.

Toxicity testing using the isolated in vitro perfused ovary

This article discusses the use of in vitro perfusion techniques as a tool for toxicity testing in the ovary and how the rat ovary has been adapted for this purpose. A brief review of the development of in vitro ovarian perfusion is provided, focusing on steroidogenesis and physiology of ovulation. Adaptation of this model for use as a toxicologic model is discussed in the context of other isolated organ models, (that is, liver, heart, lung). Surgical procedures, perfusate and criteria for viability are outlined. Advantages of this technique are highlighted including ability to administer high doses of drugs directly to intact organ devoid of other influences. Applications of this model are discussed and data from studies of glutathione depleted ovaries perfused with hexachlorobenzene (HCB) are presented. Increased oxygen consumption after addition of HCB is suggestive of a disordered respiratory metabolism and is an example of future markers of ovarian injury using this innovative technique.

Developmental anomalies derived from exposure of zygotes and first-cleavage embryos to mutagens

Results of continuing studies indicate that the mouse zygote and two-cell embryo stages are a window of susceptibility in the experimental induction of congenital anomalies with certain mutagenic agents. The mechanisms by which the mutagens initiate the pathogenesis of these developmental defects are not known. However, in certain cases there is evidence that a nonconventional, perhaps epigenetic, mechanism is involved. Detailed characterization of the spectrum of anomalies induced and comparison of responses at the various stages exposed allowed classification of the mutagens generally into two groups. One group is characterized by being effective only in the early stages of zygote development and capable of producing a relatively high incidence of fetal death and hydrops. The other group affects all of the zygote stages studied as well as the two cell-embryo, but does not increase the incidence of fetal death and hydrops. Except for hydrops, chemicals in the two groups do not differ in terms of the types of anomalies present among malformed live fetuses, which bear a resemblance to a subset of common, sporadic human developmental anomalies that are of unknown etiology. This similarity raises the possibility that certain human developmental defects may have their origins in events that happen in the zygote and early pre-implantation stages.

Developmental response of zygotes exposed to similar mutagens

Exposure of mouse zygotes to ethylene oxide (EtO) or ethyl methanesulfonate (EMS) led to high incidences of fetal death and of certain classes of fetal malformations (Generoso et al., 1987, 1988; Rutledge and Generoso, 1989). These effects were not associated with induced chromosomal aberrations (Katoh et al., 1989) nor are they likely to be caused by gene mutations (Generoso et al., 1990). Nevertheless, the anomalies observed in these studies resemble the large class of stillbirths and sporadic defects in humans that are of unknown etiology, such as cleft palate, omphalocoel, clubfoot, hydrops and stillbirths (Czeizel, 1985; Oakley, 1986). Therefore, we continue to study the possible mechanisms relating to induction of these types of zygote-derived anomalies in mice. Effects of zygote exposure to the compounds methyl methanesulfonate (MMS), dimethyl sulfate (DMS), and diethyl sulfate (DES), which have similar DNA-binding properties as EtO and EMS, were studied. DMS and DES, but not MMS, induced effects that are similar to those induced by EtO and EMS. Thus, no site-specific alkylation product was identifiable as the critical target for these zygote-derived anomalies. We speculate that the developmental anomalies arose as a result of altered programming of gene expression during embryogenesis.

Embryologic and cytogenetic effects of ethanol on preimplantation mouse embryos in vitro

Ethanol and its primary metabolite acetaldehyde were studied in cultured preimplantation mouse embryos with respect to embryotoxicity, embryolethality, chromosome breaking activities, and ability to induce sister chromatid exchange (SCE). Analysis of differentiation and cell number of mouse morulae and blastocysts show that acetaldehyde is three orders of magnitude more toxic than ethanol, indicating that the metabolite is responsible for the embryotoxicity of ethanol in preimplantation embryos. Concentrations of ethanol that do not inhibit growth induce SCEs and chromosome aberrations. The SCE-inducing effect of ethanol disappears in the presence of 4-methylpyrazole (4-MP), an inhibitor of alcohol dehydrogenase (ADH). These data suggest that preimplantation embryos are able to convert ethanol to acetaldehyde and that ADH is the enzyme involved. It is, furthermore, shown histochemically that mouse oocytes as well as morulae and blastocysts are able to oxidize ethanol in the presence of NAD+.

Induction of fetal malformations after treatment of mouse embryos with methylnitrosourea at the preimplantation stages

The effects of methylnitrosourea (MNU) on the development of preimplantation mouse embryos were investigated in this study. ICR mice were treated intraperitoneally with single doses of 10, 20, and 30 mg MNU/kg body wt on day 0, 1, 2, or 3 of pregnancy. The uterine contents were examined on day 18 of pregnancy. The fetuses were examined for external and skeletal abnormalities. No significant differences were observed in the number of implantation sites between all the MNU-treated groups and controls. MNU treatment on day 2 or 3 of pregnancy caused dose-dependent significant increases in the incidence of abnormal fetuses over the control level, while treatment on day 0 or 1 failed to cause an increase of abnormalities. Cleft palate, exencephalus, and malformed vertebrae were the most common types of abnormalities. In the embryo transfer experiments, the frequency of fetal abnormalities induced when embryos were transferred from MNU-treated females to untreated pseudopregnant females was significantly higher than that induced when embryos were transferred from untreated females to MNU-treated or untreated pseudopregnant females. The results in the present study confirm and extend the previously proposed hypothesis that the direct effects of MNU on preimplantation embryos make a significant contribution to the induction of fetal malformations.

Occupational ethylene oxide exposure and reproduction

Animal and epidemiological studies on the reproductive toxic effects of ethylene oxide (ETO) were considered in relation to occupational exposure levels (OELs) of ETO in the occupational environment of sterilisation units. Actual exposure levels in sterilisation units at Belgian and Dutch hospitals are presented and compared to data from recent studies conducted elsewhere. The animal studies did not match the actual exposure situation, involving a pattern of high peak levels and low time-weighted average levels. This may be the reason why epidemiological studies show contrasting results; they suggest reproductive toxicity of ETO at actual exposure levels. However, human data are scarce. There is a need for animal studies with a design that reflects the actual exposure situation. Epidemiological studies on reproductive events are also needed and a multi-country study would seem to be a possible approach, provided that the study design and data collection method are standardised.

Fetal pathology produced by ethylene oxide treatment of the murine zygote

Exposure of female mice to ethylene oxide by inhalation 1 or 6 h after mating produced not only multitemporal death of conceptuses but also high rates of abnormalities among surviving fetuses. In contrast, only marginal effects were observed when females were exposed 9 or 25 h after mating. The abnormalities found among 17 day gestation live fetuses were predominated by hydrops and eye defects, which, together, constitute 54% of all anomalies. Most of the remaining anomalies were distributed among 5 other types: small size, cleft palate, and cardiac, abdominal wall, or extremity and/or tail defects. In a follow-up study, the fetuses of females treated 6 h postmating were examined at 11-15 days gestation and the progression of fetal death and of malformations was studied. Results indicate that the expression of most fetal anomalies does not become apparent until late in gestation. Several of these induced anomalies are similar to common human sporadic birth defects. This new class of experimentally induced fetal anomalies provides a new avenue for investigating zygotic biology and a system for studying the progression of aberrant development.

Chromosome malsegregation and embryonic lethality induced by treatment of normally ovulated mouse oocytes with nocodazole

The mouse egg is ovulated with its nucleus arrested at the metaphase-II stage of meiosis. Sperm entry triggers the completion of the second meiotic division. It has been speculated that damage to the meiotic spindle of normally ovulated eggs at around the time of sperm entry could result in chromosome malsegregation and the death of conceptuses with numerical chromosome anomalies. This hypothesis was tested using nocodazole, a microtubule inhibitor. Nocodazole was administered either to maturing preovulatory oocytes or to normally ovulated eggs at one of the following stages: (1) the time of sperm entry, (2) early pronuclear stage, (3) pronuclear DNA synthesis, (4) prior to first cleavage division, (5) early 2-cell stage, or (6) prior to the second cleavage division. Little or no effect was observed for treatment times other than the time of sperm entry, when the egg is being activated to complete the second meiotic division. Remarkably high frequencies of embryonic lethality, expressed at around the time of implantation, were induced at this stage. Cytogenetic analysis of first cleavage metaphases of zygotes treated at the time of sperm entry revealed a high incidence of varied numerical chromosome anomalies, with changes in ploidy being predominant.

Fetal anomalies produced subsequent to treatment of zygotes with ethylene oxide or ethyl methanesulfonate are not likely due to the usual genetic causes

Earlier studies in this laboratory revealed that ethylene oxide (EtO) or ethyl methanesulfonate (EMS) induced high frequencies of midgestation and late fetal deaths, and of malformations among some of the surviving fetuses, when female mice were exposed at the time of fertilization of their eggs or during the early pronuclear stage of the zygote. Effects of the two mutagens are virtually identical. Thus, in investigating the mechanisms responsible for the dramatic effects in the early pronuclear zygotes, the two compounds were used interchangeably in the experiments. First, a reciprocal zygote-transfer study was conducted in order to determine whether the effect is directly on the zygotes or indirectly through maternal toxicity. And second, cytogenetic analyses of pronuclear metaphases, early cleavage embryos, and midgestation fetuses were carried out. The zygote transplantation experiment rules out maternal toxicity as a factor in the fetal maldevelopment. Together with the strict stage specificity observed in the earlier studies, this result points to a genetic cause for the abnormalities. However, the cytogenetic studies failed to show structural or numerical chromosome aberrations. Since intragenic base changes and deletions may also be ruled out, it appears that the lesions in question induced in zygotes by the two mutagens are different from conventional ones and, therefore, could be a novel one in experimental mammalian mutagenesis. Alternatively, the mechanism could involve a non-mutational 'imprinting' process that caused changes in gene expression.

Cytogenetic studies on preimplantation mouse embryos exposed to methylnitrosourea in vivo

Since exposure of mice to methylnitrosourea (MNU) during the preimplantation period can induce malformations and an increased postnatal death rate, direct embryotoxic effects were studied in preimplantation embryos shortly after treatment of pregnant mice on days 2 and 3 of gestation with single i.p. injections of 2.5, 5.0, and 10.0 mg/kg MNU. Embryos exposed to MNU for 24 h after treatment on day 2 showed a significant reduction of cell number and induction of sister chromatid exchange (SCE) frequency, but no structural chromosomal aberrations or inhibition of development during culture. Embryos exposed to MNU in vivo for 3 h on day 3 showed significantly reduced cell numbers, a significant inhibition of development in culture, and an increase in structural chromosome aberrations. Due to the high cytotoxicity of MNU, determination of SCE was not possible. The results indicate that MNU reaches preimplantation mouse embryos shortly after maternal treatment and that malformations seen at term and postnatal effects are probably induced by the direct action of MNU on early embryos. Furthermore, the importance of the time interval chosen for evaluation of toxicologic endpoints in preimplantation embryos is demonstrated.

Abnormal development of mouse embryos exposed to methylnitrosourea before implantation

On day 2 of gestation mice were exposed to single i.p. injections of 5, 10, 20, and 40 mg/kg methylnitrosourea (MNU). Evaluation at term revealed 100% embryolethality in the 40 mg/kg group but no signs of maternal toxicity (LD50 = 400 mg/kg). In mice treated with 5 and 10 mg/kg, no malformations could be detected at term. In contrast, 40% of the live fetuses exposed to 20 mg/kg MNU showed developmental abnormalities of vertebrae, ribs, long bones, and kidneys. Analysis of postnatal development 3 weeks after birth indicated a significant increase in mortality in the offspring of all animals exposed to MNU on day 2 of pregnancy. Further developmental or morphologic anomalies could not be detected in the offspring up to the age of 6 months, when autopsy was performed. The data show that exposure to MNU before implantation has embryolethal and teratogenic effects in a dose range one order of magnitude lower than the toxic dose range for adult animals.

Increased chromosome aberration levels in cells from mouse fetuses after zygote X-irradiation

Cell cultures derived from skin biopsies of mouse fetuses X-irradiated at the zygote stage (2 Gy) were studied for the presence of chromosomal aberrations and micronuclei. In comparison with cells from control, unirradiated fetuses significantly higher spontaneous frequencies of aberrations per 100 metaphases and aberrant metaphases were found in cells which were obtained from normal fetuses and fetuses with gastoschisis after irradiation of the zygote. The proportion of aberrant metaphases in cells from fetuses with gastroschisis but not the frequency of aberrations per 100 cells was significantly higher than in cells from normal fetuses. Spontaneous aberrations were mainly chromatid and chromosome fragments. A significantly higher proportion of cells with micronuclei was observed in cells from fetuses with gastroschisis than in cells from control or normal fetuses after irradiation. The induction of chromosome instability in fetal fibroblasts after zygote irradiation has not previously been reported.

Mouse germ cell mutation tests in genetic risk evaluation of chemical mutagens

That certain environmental chemicals can induce transmissible mutations in germ cells of experimental mammal is clear. The assumption that under certain conditions these chemicals are also likely to be mutagenic to human germ cells is not detectable. However, it is a difficult challenge to determine the level of human exposure at which such chemicals can be produced and used economically without significantly harming human health. Data on transmitted genetic effects in mice are necessary, not only as a measure of endpoints that are considered directly in genetic risk assessment, but also as the standard for evaluating the usefulness of non-germ-cell effects as predictors in genetic risk assessment. To carry out a "real world" genetic risk assessment exercise, in vivo mouse data are being obtained for two model chemicals--ethylene oxide and acrylamide. Both chemicals are capable of inducing transmissible genetic effects in mice; their production and use involve measurable human exposures; and, because they are socially and economically important, they are not likely to be banned altogether despite their mutagenicity. For both chemicals, data are not sufficient for accurate low-dose and low-dose-rate extrapolations.

High frequency of mosaic mutants produced by N-ethyl-N-nitrosourea exposure of mouse zygotes

Mouse zygotes containing one multiple-recessive parental genome (a, b; p cch; d se; s) and the corresponding wild-type alleles in the other were exposed to N-ethyl-N-nitrosourea (ENU) at various stages in vivo. At weaning age, the resulting mice were examined for mutations at the marked loci as well as at others producing externally visible phenotypes. Because of viability problems in one of two reciprocal crosses, the bulk of the mutagenesis data are derived from the cross that detects recessive mutations in the maternal genome. The mutation rate was approximately 8 times higher in groups treated 2.5-3 hr postmating (sperm entry, completion of second meiotic division) than in those injected 5-6 hr postmating (pronuclear formation). In the former more sensitive zygote population, the mutation rate is about an order of magnitude greater than that induced by the same ENU exposure (50 mg/kg) to spermatogonial stem cells. Of 11 mutants recovered, 8 were mosaics. Progeny tests have demonstrated germ-line involvement for most of the mosaics, and the average fraction of the germ line carrying the mutation is close to 50%. The nature of the mutations indicates (i) that the mosaicism results not from misassortment at the first cleavage but from mutation affecting one DNA strand of the maternal chromosome, and (ii) that the mutations are intragenic lesions rather than multilocus deletions, thus resembling ENU-induced mutations in spermatogonia. The finding that mosaicism for presumed point mutations is readily inducible by ENU treatment of zygotes may provide a means of generating genetic materials that can be of use for developmental studies.

Mutagen-induced fetal anomalies and death following treatment of females within hours after mating

In an earlier study (Generoso et al., 1987), it was observed that the mutagen, ethylene oxide (EtO), produced remarkable increases in the incidence of developmental abnormalities and death of fetuses when early zygotic stages were exposed. This is a major finding in experimental induction of embryopathy, implicating genetic damage to the zygotes as the likely cause. In the subsequent study reported here, 3 other mutagens--ethyl methanesulfonate (EMS), ethyl nitrosourea (ENU), and triethylene melamine (TEM), were studied for embryopathic effects following exposure of dictyate oocytes, prefertilization oviducal eggs and sperm, early pronuclear zygotes, zygotes undergoing pronuclear DNA synthesis, and two-cell embryos. All 4 mutagens produced developmental abnormalities among living fetuses following exposure of early pronuclear zygotes (the only stage studied for this endpoint in this report). With respect to stage specificity and gestational timing of death of conceptuses, EMS and EtO on one hand and ENU and TEM on the other, are very similar to one another. EMS, like EtO, produced a high incidence of midgestation and late fetal deaths only in prefertilization oviducal eggs and sperm and in early pronuclear eggs. In contrast, ENU and TEM produced high losses of conceptuses in all postmating stages studied but death occurred primarily prior to or around the time of implantation. Thus, the frequency of induction and the expression of embryopathy, which ranged from early embryonic preimplantation and late fetal deaths to subtle fetal anomalies, are dependent upon the stage exposed and the mutagen used.