Background
The adverse effects caused by various natural and synthetic chemicals include impairment of both the male and the female reproductive system. Many of these effects are related to temporary or permanent actions on the endocrine system. When the mechanisms of action of these chemicals are endocrine-mediated, they may induce non-heritable defects culminating with sub/infertility, growth retardation, endocrine disorders or even death of the organism/subpopulation or its descendants [
1].
Subfertility of women at reproductive age is often caused by endocrine disorders, occasionally (~10% of the cases) characterized by polycystic ovary syndrome (PCOS) [
2,
3]. Although there is strong evidence that PCOS is a genetic disease, diagnoses do not always indicate the cause of the sub/infertility neither the mode of action of the causative agent [
4]. It has been shown that PCOS can be correlated with exposure to the endocrine disruptor bisphenol A (BPA) [
5]. Also, women with PCOS when exposed to nicotine have a higher chance to acquire other metabolic disorders [
6]. Probably, many other endocrine active substances present in the environment, food, drugs or cosmetics will also cause subfertility. More than 25% of the subfertility cases are of unknown etiology and remain not diagnosed, and the affected couples attempt to achieve their parenthood experience through ART. However, knowledge on the source of subfertility, as well as the risks of oocyte defects during maturation will improve the success of IVF programs. Women exposed to a determined toxic agent may have a temporary negative effect on their oocyte quality, which may reflect on the health of the offspring.
Most information regarding reproductive toxicology has been obtained from case reports, clinical analyses and
in vivo tests with mice and rats. The use of laboratory animals for such tests is under debate for many years [
7], because
in vivo reproductive toxicity tests require large numbers of animals. Indeed approximately 70% of all the animals used in toxicological studies are for assays involving reproductive toxicity [
8]. Furthermore, rodents are not the most suitable model animals for human, especially when considering oocyte maturation and fertilization [
9].
Although the
in vivo assays should not as yet be eliminated, alternative assays including computational, integrative
in vitro tests using embryonic stem cells and cell lines are novel approaches in reproductive toxicology [
10,
11]. Available screening tests for hormone-like active compounds do not identify endocrine disruptors nor assure that a chemical will have an endocrine activity [
12]. Nevertheless, such tests are useful to suggest the potential effect of certain substances. Furthermore, if the tests are integrated and projected with a clear knowledge on the main targets of reproductive toxic compounds and their involved mechanisms, the use of a large number of laboratory animals can be avoided.
Due to their complexity, not always reproductive cells such as oocytes can be mimicked by somatic cells. Moreover, oocytes and their surrounding cumulus cells have to undergo a unique process known as maturation that for the oocyte includes meiosis. Indeed, during maturation, oocytes are susceptible to epigenetic alterations that may interfere with fertilization and early embryo development [
13]. Accordingly, exposure to polycyclic aromatic hydrocarbons before ART affects oocyte quality, as observed in women with a lower rate of cell division after IVF [
14]. By evaluating data from different laboratories, it has been suggested that maturation of bovine oocytes can be used as a reliable model to screen toxic agents for human oocytes [
15‐
17]. Also, the use of the porcine model has been indicated to evaluate oocyte maturation as a model for human oocytes due to some similarities between these species [
18‐
20]. Furthermore, both bovine and porcine models can reduce the large number of laboratory animals used for reproductive testing, since oocytes can be obtained from slaughterhouse ovaries which are leftover organs when animals enter the food production chain.
The REACH (Registration, Evaluation, and Authorization of Chemicals) legislation is a European program impacting manufacturers worldwide since 2007/2008. For instance, marketing in Europe of any chemical (pure form or in a formulation) is allowed solely if data on physical/chemical properties, toxicity and environmental effects are provided. As a result, there is an enormous increase in the number of experimental animals used to test the safety of many thousands of chemicals, existing and new ones [
21]. However, the adopted EU directive from 2010 focus on the protection of animals used for scientific purposes and demands a decrease in the number of animals used in research, including toxicity (drug) testing [
22]. Innovative approaches have been suggested by developing a battery of tests targeting aspects of the reproductive cycle, and by integrating the approaches based on the mechanisms in cells and tissues [
21]. The possibility to use slaughterhouse material for
in vitro tests related to gametes and early embryo development appears as an important option to diminish the number of
in vivo tests.
The aim of the present review was to evaluate the usefulness of using bovine and porcine IVM/IVF as model for reprotoxicity studies. For this, we address reproductive toxins and toxicants and their main effects on female fertility focusing on in vitrooocyte maturation and fertilization. Moreover, the use of oocytes from cattle and pig provides insight on the possibilities to evaluate chemicals on the in vitro oocyte maturation, fertilization and early embryonic development (preimplantation stage) as a model for human.
Many couples apply to ART because of fertility problems and more than one fifth of the causes of sub/infertility in female are unknown. There is compelling evidence that fertility problems are related to the exposure to different sources of contaminants, among them dietary and environmental appearing as a great concern. Some of the identified chemicals can affect the oocyte already during maturation. If the oocyte does not follow an apoptotic pathway, there is still the risk that fertilization and subsequent embryo development will be somehow affected. Furthermore, when ART is necessary, patients should be advised about the environmental and dietary contaminants not only because of metabolic diseases, but also to prevent exposure to hazardous chemicals.
To identify the effect of such dietary and environmental compounds, cell lines have been used as an important screening tool. However, a cell line will not always mimic gametes during a critical phase of maturation, which requires specific biochemical changes. It is possible to apply reliable and more complex screening methods, as well as decrease the number of laboratory animals, by using bovine or porcine as models for IVM, IVF and embryo production in vitro. Obviously the bovine/porcine model will not replace rapid and massive screenings with cell lines, and cannot completely eliminate in vivo studies with rodents. Although, the research material will be obtained from healthy animals that entered the food chain and no extra facilities are necessary to house animals, bovine and porcine IVM/IVF models have certain disadvantages that should be considered when applied to test potential toxic compounds. For instance, the origin of slaughterhouse-derived materials (ovaries) is often unknown such as age, reproductive status, possible reproductive disorders or stressful conditions. To overcome this, Ovum Pick-Up (OPU) from donors with known background can provide a solution. Even though cattle oocyte development is rather similar to that of humans, in in vivo studies the porcine appears as a good model for transgenerational research since it is a monogastric species with a gastrointestinal tract similar to that of humans. Overweighing supposed risks of certain chemicals should be avoided. However, human beings are daily exposed to different contaminants. Even though exposure may occur at extremely low concentrations of each contaminant, chronic multi-exposure should be taken into account, and epigenetic changes affecting embryo development must be considered.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
RRS and EJS participated in the survey of the data. RRS, EJS and BAJR conceived the study, and participated in its design and wrote the manuscript. All authors read and approved the final manuscript.