Microplastics, often abbreviated as MPs, are small plastic particles under 5 millimeters that are now widely recognized as an environmental issue. They have made their way into our water systems and the broader food chain, prompting extensive studies on how they might affect reproduction in different species, humans included. This article explores the possible ways in which microplastics could be affecting reproductive health and fertility.

Oxidative stress

Oxidative stress is a cellular phenomenon that happens when there’s a mismatch between the creation of free radicals, like reactive oxygen species (ROS), and the body’s antioxidant defenses that neutralize their damaging effects This mismatch can cause harm to cells and tissues, playing a role in various diseases and the aging process. (Pizzino et al., 2017).

When it comes to microplastics (MPs), oxidative stress is a major issue. MPs can cause oxidative stress by interfering with the cells’ electron transfer processes, which results in an excess of ROS. These ROS can harm important cellular components such as lipids, proteins, and DNA, all vital for cell health and function (Abdal Dayem et al., 2017). For example, research has indicated that MPs exposure can increase ROS in oyster sperm, leading to reduced fertilization success. Likewise, studies on rats have shown that polystyrene MPs (PS-MPs) can induce oxidative stress in ovarian cells, affecting ovulation (Ferrante et al., 2022)

Hormonal Havoc: Disrupting the HPG Axis

The Hypothalamic-Pituitary-Gonadal (HPG) Axis is an essential hormonal system that controls reproductive functions. It’s a network involving the hypothalamus, pituitary gland, and gonads (ovaries or testes), which are responsible for producing and regulating sex hormones vital for reproductive health. (Mikhael et al., 2019)

Microplastics (MPs) have been identified as disruptors of the HPG Axis, causing hormonal imbalances that could impact fertility. Research has indicated that exposure to polystyrene MPs (PS-MPs) in male mice can lower testosterone levels and affect the balance of other important hormones such as follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormonal alterations can hinder the growth and maturation of reproductive organs, adversely affecting the development of offspring (Zhang et al., 2022). In female mice, exposure to MPs has resulted in similar hormonal disturbances, with changes observed in FSH and estradiol levels. (Liu et al., 2021)

Regarding reproductive success, it’s known to be an energy-demanding process that relies on ample resources for the creation of gametes, fertilization, and embryo growth. MPs pose a threat to an organism’s energy metabolism, which can influence reproductive success.

Evidence suggests that MPs can modify the patterns of food intake and energy distribution within organisms, which may lead to a decrease in reproductive output (Jewett et al., 2022). For example, studies on oysters have shown that MPs can reduce the activity of enzymes that are crucial for energy production during sperm development. This reduction could lead to sperm quality issues and, consequently, affect the viability of the offspring. (Sussarellu et al., 2016)

Microcirculation Woes: A New Frontier

Microcirculation is the process of blood flow through the body’s tiniest vessels, like capillaries. It’s crucial for supplying tissues with nutrients and oxygen and for eliminating waste. Any interference with microcirculation can greatly affect reproductive health.

Recent research has shown that microplastics (MPs) can negatively impact microcirculation. This can cause developmental issues and raise the mortality rate of embryos in water-dwelling species such as zebrafish. For instance, exposure to MPs and nanoparticles (NPs) has been linked to microcirculation damage, especially in vital areas like the tail, which is important for proper growth. (Zhang et al., 2022)

Human Health Implications

The increasing concern about microplastics (MPs) and their effect on human reproductive health is quite substantial. There has been a noticeable decrease in male semen quality over the last 80 years, and environmental pollutants, including MPs, are suspected to be contributing factors. Insights from animal studies have highlighted potential mechanisms through which MPs may influence health, such as oxidative stress, inflammation, and hormonal disturbances.

Direct research on the impact of MP exposure on male infertility in humans is not yet available, but animal studies have suggested a minimum human equivalent dose of MPs that could result in poor semen quality. This dose is estimated to be 0.016 mg/kg/day. The proximity of this figure to the levels of MP exposure observed in some countries points to a possible threat to human reproductive health. It underscores the importance of ongoing research to fully understand the effects and to develop appropriate exposure guidelines.

Conclusion

The research collectively points to a considerable risk that microplastics (MPs) present to reproductive health in various species. It’s vital to grasp the damaging mechanisms—like oxidative stress, hormonal imbalance, energy shortage, and microcirculation problems—to devise ways to lessen their effects. With MPs increasingly becoming a part of our environment, it becomes more pressing to confront their reproductive consequences. This calls for thorough policy-making and additional studies to safeguard human health. 

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