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

Micro- and nanoplastics, defined broadly as plastic particles less than 5 millimeters and down to the nanometer scale, have become ubiquitous in marine, terrestrial, and atmospheric environments. Their generation via top-down processes — mechanical breakdown, weathering, and other physical disintegration of larger plastic debris — creates a complex milieu of particles varying in size, shape, and chemical composition. This diversity complicates the assessment of their biological impact, yet the study led by van den Berg, Adriaans, Parker, and colleagues meticulously investigates how these particulates interact specifically with macrophages, the frontline defenders of our innate immune system.

Macrophages play an indispensable role in immune surveillance and homeostasis by engulfing pathogens, dead cells, and foreign particles through phagocytosis. Disruptions in macrophage function can lead to impaired immune responses and tissue homeostasis. The research team employed in vitro models to expose macrophages to carefully characterized micro- and nanoplastic particles generated via top-down methods, examining cellular viability, immune activation markers, and inflammatory responses over various exposure durations and concentrations.

One of the most striking discoveries reported is that exposure to these plastics significantly reduces macrophage viability. Quantitative assays demonstrated a dose-dependent decrease in viable macrophage populations, indicating cytotoxic effects that could compromise the ability of immune cells to perform critical functions. This cytotoxicity was consistent across different particle sizes but appeared more pronounced with smaller nanoplastics, suggesting size-dependent cellular interactions and internalization dynamics.

However, perhaps more surprising was the observation that despite evident cytotoxicity, these plastic fragments did not elicit a classical pro-inflammatory response. Typically, foreign particles trigger macrophages to upregulate inflammatory cytokines such as TNF-alpha, IL-6, and IL-1β, signaling an immune alarm that recruits other immune effectors. In this study, the macrophages exposed to micro- and nanoplastics showed minimal induction of these cytokines, indicating a muted inflammatory signaling cascade. This paradoxical finding raises complex questions about the immunomodulatory effects of plastic particulates.

The muted inflammatory response could be interpreted as a form of immune evasion; the plastics induce macrophage death without activating defensive signaling, potentially allowing these particles to persist undetected within tissues. Detailed mechanistic probing revealed that the plastic particles might interfere with intracellular pathways responsible for inflammation, possibly via physical disruption of cell membranes or the sequestration of signaling molecules.

Moreover, advanced imaging techniques employed by the researchers provided evidence of internalization of these particles into macrophages, with localization primarily within lysosomal compartments. This suggests that macrophages are actively engulfing micro- and nanoplastics, but the subsequent intracellular fate of these materials might contribute to cellular stress or toxicity without initiating canonical danger signals. The chronic impact of such intracellular accumulation remains a critical avenue for future investigation, especially considering potential implications for diseases linked to impaired immune clearance.

The implications of these findings are far-reaching. If macrophage viability is reduced in vivo due to environmental exposure to top-down generated micro- and nanoplastics, systemic immune defense mechanisms could be undermined, potentially increasing susceptibility to infections and disrupting tissue regeneration processes. Additionally, the lack of an appropriate inflammatory response might facilitate the silent accumulation of plastics within various organs, contributing to long-term pathological sequelae that are yet to be fully characterized.

Environmental scientists and toxicologists alike have long debated the relative risks posed by primary microplastics, engineered at the nanoscale, versus secondary plastics derived from environmental fragmentation. This study adds compelling evidence highlighting that top-down generated particles, often overlooked, have unique and insidious effects on immune cells that differ from those generated by bottom-up synthetic processes.

Importantly, the research methodology integrated rigorous particle characterization using techniques such as scanning electron microscopy (SEM), dynamic light scattering (DLS), and Fourier-transform infrared spectroscopy (FTIR), ensuring precise identification of particle size distribution and chemical signatures. This robust approach allows for reproducibility and aids in the broader application of findings to environmental health risk assessments.

The study also underscores the necessity of revisiting current regulatory frameworks governing micro- and nanoplastic pollution. Traditional assessments focusing mainly on overt inflammatory and cytotoxic outcomes could underestimate the subtle immunosuppressive or stealth toxicity mechanisms now recognized as critical. This gap demands integrated interdisciplinary research efforts bridging environmental chemistry, immunology, and toxicology.

From a public health perspective, the findings amplify concerns regarding the human exposure pathways to micro- and nanoplastics through inhalation, ingestion, and dermal contact. Macrophages reside not only in systemic circulation but also in lung tissue, gut mucosa, and skin, implicating multiple organ systems in the potential adverse effects of plastic infiltration. Researchers advocate for longitudinal epidemiological studies to link environmental plastic exposure with immune system dysfunctions.

Beyond immediate immune impacts, the study invites deeper inquiry into downstream biological consequences. For instance, impaired macrophage viability might affect antigen presentation and adaptive immunity, potentially compromising vaccine responses or facilitating autoimmunity. The absence of inflammatory signaling might also permit plastics to act as carriers for other environmental toxins or pathogens, exacerbating health risks through combined exposures.

This pioneering work spearheaded by van den Berg and colleagues therefore represents a critical step forward in unraveling the complex bio-nano interactions of plastics. It challenges existing paradigms that equate toxicity solely with inflammatory activation, urging the scientific community to rethink how subtle cellular disruptions can translate into broader organismal vulnerabilities.

As research continues to peel back layers of microplastic impacts on biological systems, this study sets a precedent for nuanced examination of the immune consequences resulting from exposure to fragmented plastics. Disentangling the molecular underpinnings of macrophage responses to such pollutants will be crucial for developing diagnostic markers and mitigation strategies.

Finally, this research carries a sobering message about the unintended consequences of pervasive plastic usage and pollution. The stealth toxicity of top-down generated micro- and nanoplastics compels us to accelerate efforts in reducing plastic waste, innovating biodegradable materials, and improving waste management practices worldwide to safeguard human health and ecological integrity.

Subject of Research: The impact of top-down generated micro- and nanoplastics on macrophage viability and inflammatory response.

Article Title: Top-down generated micro- and nanoplastics reduce macrophage viability without eliciting a pro-inflammatory response.

Article References:
van den Berg, A.E.T., Adriaans, K.J., Parker, L.A. et al. Top-down generated micro- and nanoplastics reduce macrophage viability without eliciting a pro-inflammatory response. Micropl.& Nanopl. 5, 32 (2025). https://doi.org/10.1186/s43591-025-00138-5

Image Credits: AI Generated