The Silent Threat of Microplastics

Photo by Brian Yurasits on Unsplash

Plastics. Cheap, useful, and omnipresent. From toys made for children to the disposable bottles used by millions daily, these materials have become one of the core building blocks of our society. As time goes on, the health risks posed by these items have been put into question time and time again. It has become evident that they have found their way into our bodies as well, this time in the form of microplastics, tiny shards of the material less than 5 millimeters wide. However, the risks posed by microplastics are still unclear, as research in the field is only in its infancy, and the long-term effects of microplastics on humans are just beginning to appear. As shown in the 2023 study Acute Exposure to Microplastics Induced Changes in Behavior and Inflammation in Young and Old Mice however, such risks can be monitored through the use of lab mice, which have much shorter life spans than humans.

The study itself was divided into three experiments: cell viability, behavioral, and post-mortem bioaccumulation experiments.

The first experiment tested cellular reuptake and viability. Cellular reuptake is the process of a secreted substance being reabsorbed back into the cell that created it, while cellular viability is the health of a general sample of cells. After being exposed to microplastics in vitro, meaning that cells were isolated and then experimented on. Cells were exposed to “commercially available pristine fluorescent polystyrene particles”. Polystyrene is a type of plastic widely used in packaging products, such as bottles, meat trays, and styrofoam. This was done in equal doses but in varying concentrations across a 72-hour period. Cell viability was measured at each 24-hour mark during the timespan, the researchers noting that “after 48 and 72 h, PS-MPs of both sizes induced a dramatic decrease in cell viability”. Furthermore, the negative shift in cell viability featured a strong correlation with the concentration of microplastics.

According to the researchers, decreased cell viability signals an alarming risk of microplastics in cells, as their experiment suggests “PS-MPs in this size range exhibit cytotoxicity”. The ability of the microplastics to gather around the nucleus of cells was also noted, both findings further substantiating preconceived risks of microplastics. Furthermore, the relative abundance of PS-MPs heightens these risks, as their being commonplace allows for more frequent exposure with humans.

Gaspar et al. performed a second experiment, this time on female lab mice of varying ages. Mice were sorted into two groups: A young and an old group, with mice in the young group being approximately 4 months old and mice in the old group being approximately 21 months old. Afterward, the researchers separated each of the mice in both groups into control, low, medium, and high exposure groups, each group receiving a different concentration of PS-MPs in their water supplies. Researchers monitored microplastic concentrations, water consumption, and body weight, at 10-hour intervals throughout the trial to ensure homogeneity.

After a 3-week exposure period, mice from all groups were given behavioral testing, in which mice were given time to roam around a low-lit chamber for an hour and a half, with researchers recording all spontaneous movements. In their report, researchers remarked, “Surprisingly, we found that acute exposure to PS-MPs induced an increase in distance traveled, which was more pronounced in older animals”. In a follow-up experiment performed by researchers, mice were placed into a chamber that was divided into light and dark zones. Researchers once again observed for any spontaneous movements. This experiment once again confirmed the findings of the low-lit chamber experiment, as mice with acute exposure traveled the most distance, the side effects being most prominent in older mice. Through this, the behavior changes associated with microplastic exposure become evident, further posing a threat to humans.

The final experiment performed by the researchers was the examination of cryosectioned tissue from parts of the mice. Using counterstaining in order to see the microplastics within the tissues, researchers "detected PS-MPs within intracellular compartments of every tissue examined”. This is evidenced in the graphic below, in which the pink specks are pieces of PS-MP inside of each piece of tissue. These tissues ranged from the livers and hearts to most concerningly, the brains of mice from both age groups. The discovery of PS-MPs in organs such as the heart and lungs suggests that they are able to penetrate through the digestive system and into the circulatory system, thus implying their ability to take part in systemic circulation. Furthermore, PS-MPs being found in the brain show their ability to penetrate the blood-brain barrier, which raises potential concerns about their impact on the brain. 

Tissue with microplastics detected through counterstaining.

Inside the brains of the mice, researchers attempted to analyze the concentration of GFAP within brain cells. GFAP, short for glial fibrillary acidic proteins, is an important marker within the brain that signals the presence of glial cells, which are vital in aiding neurotransmitter uptake, the process in which neurons share information. Autophagy is the process by which the cell destroys old substances and proteins within the cytoplasm and within surrounding cells. A decrease in GFAP would mean that fewer glial cells are available in order to help enable these vital functions. Researchers noted that mice exposed to PS-MPs showed decreased GFAP expression, with this decline being especially pronounced in young mice. Decreased GFAP expression in humans is often correlated with depression in younger patients and Alzheimer's in older patients, presenting a frightening risk for our species. 

On a cellular level, the ability of microplastics to poison and reduce the viability of cells raises concerns not only for organisms grown in Petri dishes but for entire ecosystems, as wild animals unknowingly sustain themselves on poisoned earth. However, even without the consideration of the world around us, the tendency for microplastics to penetrate the blood-brain barrier signals an alarming change in global health trends, as through their experiments on rodents, Lauren Gaspar and her colleagues show the risk of microplastics on animals. Animals whose neurological makeup almost mirrors ours in similarity. Animals whose lives have been sacrificed for progress that our species may never capitalize on—unless we confront the silent threat of microplastics.