The study was designed to address a glaring gap in the microplastics literature. Although microplastics have been confirmed in human tissues, including the liver, intestines, and even the placenta, most toxicological investigations have relied on in vitro cell cultures or single-sex animal cohorts, leaving regulators with limited information about how whole organisms respond to realistic, repeated exposure. The Health Canada team chose C57BL/6 mice, one of the most thoroughly characterized laboratory strains, and divided animals by sex to capture biological variation that shorter or single-sex studies routinely miss. Crucially, they also compared two particle geometries: uniform commercial spheres, which dominate the literature because they are easy to characterize, and milled particles with jagged, irregular edges, which more closely resemble the fragmented plastics found in food, water, and air.

Animals received the particles by daily oral gavage for 28 consecutive days, a duration that in regulatory toxicology is classified as a subacute exposure window, long enough for effects to emerge in rapidly dividing tissues such as the gut lining, but short enough to isolate direct toxicity from age-related changes. The researchers then assembled an unusually comprehensive assessment battery. They tracked food consumption and body weight throughout the exposure, weighed organs at necropsy, ran complete hematology panels, measured a broad suite of clinical biochemistry markers reflecting liver and kidney function, profiled both innate and adaptive immune cell populations by flow cytometry, quantified pro-inflammatory cytokines, and examined tissues histologically under the microscope. Raman spectroscopy was used to hunt for plastic particles in collected tissues, confirming whether the material actually crossed from the gut lumen into the body.

The first surprise was how quiet the systemic picture looked. Across all treatment groups, there were no significant changes in food consumption, body weight, or relative organ weights. Hematological parameters, including red and white blood cell counts, remained within normal ranges, and clinical biochemistry markers of organ function showed no statistically meaningful disturbances. For anyone who has followed alarmist headlines about microplastics, the results might seem anticlimactic. But the team emphasizes that the absence of classical systemic toxicity endpoints does not mean the exposure was biologically inert. Subtle changes were quietly accumulating in tissues that routine blood panels cannot see, and those changes were distributed unevenly between the sexes.

The most dramatic finding emerged in the gallbladder. Histological examination revealed gallbladder inflammation in 9 of 23 male mice exposed to the particles, compared with only 2 of 24 exposed females, a skew so pronounced that it is unlikely to be chance. The inflammatory lesions appeared in animals exposed to both spherical and milled particles, suggesting that particle chemistry rather than precise geometry drove the response in this organ. The gallbladder is not a tissue that has featured prominently in microplastics research, which has focused overwhelmingly on the intestine, liver, and lungs, and the finding raises immediate questions about biliary health in chronically exposed animals and, by extension, humans. Whether the inflammation reflects direct physical contact by particles transiting the biliary tract, a secondary response to altered bile composition, or an immune-mediated process remains unresolved.

Male mice also showed a second, subtler immune signature: pro-inflammatory cytokines were decreased across all polystyrene-exposed male groups. A drop in inflammatory signaling molecules might superficially sound beneficial, but in immunology, dampened cytokine output can indicate immune dysregulation, a blunting of the normal responsiveness that animals need to fight infections and clear damaged tissue. The adaptive and innate immune cell populations measured by flow cytometry showed no major shifts, which makes the cytokine suppression even more intriguing, because it suggests a functional change in cell behavior rather than a change in cell numbers. The authors note that this pattern was specific to males; females did not display the same systemic cytokine profile.

Female mice told a completely different story, one written in the tissue of the small intestine. Instead of gallbladder inflammation, exposed females developed mild pathological changes in the intestinal lining, most notably alterations in the goblet cell population and in mucus production. Goblet cells are the gut’s dedicated mucus factories, secreting the protective gel layer that lubricates the intestinal surface and forms a critical barrier against bacteria, dietary antigens, and foreign particles. Changes in goblet cell abundance or mucus output are therefore not cosmetic; they can alter gut permeability, shift the microbiome, and modulate how the intestine tolerates everything from food proteins to pathogens. Because the intestine is the first point of contact for ingested microplastics, these findings suggest that in females, the primary biological conversation between plastic and body happens at the gut barrier itself.

Raman analysis added a crucial piece of evidence: polystyrene was detected in both the liver and the intestines of animals of both sexes. This confirms that at least a fraction of the ingested particles crossed or associated with the intestinal epithelium and reached the liver, the body’s central detoxification organ. Particle translocation is a key variable in the risk assessment of nano- and microplastics, because systemic distribution opens the door to effects far beyond the gut. The detection of plastic in liver tissue after only 28 days of exposure demonstrates that the mammalian body does not simply pass these materials through as inert cargo.

Just as important as the sex differences was what did not differ: spherical and milled particles produced broadly similar toxicological results. This is a consequential observation for the field, because much of the existing microplastics literature relies on idealized spheres for experimental convenience. If milled, irregular fragments, which better approximate environmental weathered plastic, behave like spheres in vivo, then laboratory findings using spherical particles may be more transferable to real-world risk than critics have assumed. At the same time, the authors are careful to frame the geometry question as far from settled, and the gallbladder findings appeared across both particle types in this study.

The broader takeaway is a call for sex as a standard variable in microplastics toxicology. Regulatory toxicity testing has only in recent decades moved toward requiring both sexes in study design, and the microplastics field, much of it built on expedient single-sex models, has lagged behind. The Health Canada results demonstrate that pooling or ignoring sex can mask biologically meaningful signals, from suppressed cytokine responses in males to gut barrier remodeling in females. The study was funded by the Government of Canada’s Advancing a Circular Plastics Economy Initiative, reflecting a policy landscape in which plastic production continues to accelerate globally while the health consequences of chronic, low-level human exposure remain only partially mapped. As humans continue to ingest and inhale plastic particles daily, research of this kind provides the granular, sex-disaggregated, mechanism-oriented data that will ultimately determine whether microplastics represent a quiet hazard, an active threat, or something in between.

Subject of Research: Sex-specific toxicological effects of oral polystyrene nano- and microplastic exposure in mice

Article Title: Sex-specific toxicological observations following 28-day oral gavage exposure to spherical and milled polystyrene nano- and microplastic particles in mice

Article References: Sex-specific toxicological observations following 28-day oral gavage exposure to spherical and milled polystyrene nano- and microplastic particles in mice. (n.d.). https://doi.org/10.1186/s43591-026-00226-0

Image Credits: AI Generated

DOI: 10.1186/s43591-026-00226-0

Keywords: microplastics, nanoplastics, polystyrene, toxicology, mice, sex differences, gallbladder inflammation, small intestine, goblet cells, cytokines, oral gavage, Health Canada