Microplastics can accumulate in our bodies and even reach the brain, as studies have already shown. Researchers have now examined the possible effects of this microplastic pollution on mice. Accordingly, immune cells in the brain can absorb the plastic particles. As a result, fine blood vessels became blocked and blood flow to the brain was disrupted. This led to cognitive and motor disorders in the mice. However, the extent to which the results can be transferred to humans is questionable.
Microplastics are omnipresent in our environment. The tiny plastic particles enter our bodies through the drinking water, our food and even the air we breathe. There they accumulate in various tissues, such as the lungs, liver, spleen and kidneys. Some types of microplastics have even been shown to be able to penetrate the blood-brain barrier. In this context, early signs of Alzheimer’s and depression have already been observed in mice.
Fluorescent microplastics
But how does microplastics affect the brain? To answer this question, a team led by Haipeng Huang from the Chinese Research Academy of Environmental Sciences in Beijing administered high amounts of microplastics to mice – either with drinking water or by injection. The concentrations were 0.2 milligrams per milliliter when absorbed through drinking and five to 50 micrograms per milliliter of blood in the blood. In order to trace the path of the particles in the body, the researchers marked the microplastic with a fluorescent dye. Huang and his team then looked at the blood vessels of the living mice under the microscope.
And indeed: a few hours after the mice had ingested the microplastic in the drinking water or been injected, it began to glow treacherously in the blood vessels of the brain – the fluorescent microplastic had overcome the blood-brain barrier. As the research team found, the microplastics did not circulate freely in the bloodstream, but were absorbed by blood cells, which the researchers identified as neutrophils and macrophages, i.e. phagocytes of the immune system. These cells usually absorb foreign objects such as pathogens in order to destroy them and thus protect the body. In the case of microplastics, however, this mechanism could be disastrous.
Immune cells get stuck
“Our results show that the immune cells that have ingested microplastics lead to blockages in the capillaries,” report Huang and his team. While the phagocytes usually destroy the foreign bodies they ingest, they are unable to do this with microplastics. Instead, the plastic particles make the cells less flexible and can no longer squeeze through fine blood vessels. As the researchers showed, some of the resulting blockages in the brain persisted for more than a week.
But how does this affect the behavior of the mice? In various experiments, Huang and his team observed that the mice contaminated with microplastics performed worse in memory tests and also showed motor disorders. The researchers also identified symptoms that are considered signs of depression. The researchers observed the most serious blockages and the resulting effects when they injected the mice with concentrations of six to twelve micrograms of microplastics per milliliter of blood. This concentration is also a realistic burden on people, the team writes.
Transferability to humans?
Elvira Mass from the Life & Medical Sciences Institute (LIMES) at the University of Bonn questions this statement: “The concentration of microplastics stated in the study is based on an earlier publication that has already been critically questioned and whose data are considered unrealistic,” she explains . In addition, microplastics do not enter the bloodstream of humans through injection, but through food. Smaller particles in particular manage to penetrate the intestinal barrier and enter the bloodstream. In contrast, damage to the mice was primarily caused by larger particles. In addition, the blood vessels in the mouse brain are significantly finer than in humans.
“In summary, it can be said that the transferability of the results to humans is very limited,” says Mass. “Both the unrealistic concentrations as well as the physiologically irrelevant injection method and possible contamination of the plastic particles significantly limit the significance of the study.” In order to be able to estimate the actual risks for humans, further studies are therefore necessary.
Source: Haipeng Huang (Chinese Research Academy of Environmental Sciences, Beijing, China), Science Advances, doi: 10.1126/sciadv.adr8243