Bioaccumulation of Microplastics in Decedent Human Brains – Comprehensive Study Notes

Introduction & Background

  • Rising environmental abundance of micro- and nanoplastics (MNPs) over the last 50 years → heightened concern for human exposure and toxicity.
    • MNPs defined size range: 500\,\mu\text{m}\;\text{to}\;1\,\text{nm}.
    • Previous human detections limited to large particles in lung, intestine, placenta; nanoplastics usually missed.
  • Toxicology mantra “dose makes the poison” (Paracelsus) → need for quantitative internal‐dose data to interpret animal & in-vitro findings.
  • New analytical approach: pyrolysis gas chromatography–mass spectrometry (Py-GC/MS) + orthogonal spectroscopic & microscopic methods provides more comprehensive detection, incl. blood, placenta, arteries.

Study Aims

  • Quantify and characterize MNPs in three major human organs (liver, kidney, frontal cortex of brain).
  • Compare concentrations across time (2016 vs 2024) and geography.
  • Examine polymer composition differences between organs.
  • Explore accumulation in dementia versus non-dementia brains.
  • Provide visual confirmation of particle presence, size, morphology.

Sample Collection & Demographics

  • De-identified autopsy tissues from University of New Mexico Office of the Medical Investigator (UNM OMI).
    • Timepoints: 2016 cohort & 2024 cohort; n = 20\text{–}28 per organ/time-point.
    • Organs: right-central liver, wedge kidney (cortex + medulla), frontal cortex.
  • Additional frontal cortex samples:
    • East-coast brain banks: NC (n = 13), MA (n = 9), MD (n = 5) with deaths 1997–2013.
    • Dementia cohort (UNM OMI) 2019–2024, n = 12 (Alzheimer 6, vascular 3, other 3).
  • Limited metadata: age, sex, race/ethnicity, cause & date of death; groups balanced on these variables.

Analytical Methods Overview

  • Tissue (~500 mg) digested in 10\% KOH × 3 days @ 40\,^{\circ}\text{C}.
  • Ultracentrifugation 100,000\,g\;\times\;4\,\text{h} → pellet of KOH-insoluble solids.
  • Single-shot Py-GC/MS on 1\text{–}2\,\text{mg} pellet; reference library of 12 polymers.
    • PE, PP, PVC, SBR, PS, ABS, PET, PMMA, PU, PC, N6, N66.
  • Complementary validation:
    • ATR-FTIR on select brain pellets.
    • Polarization wave light microscopy.
    • Scanning & transmission electron microscopy (SEM, TEM) + energy-dispersive X-ray spectroscopy (EDS).
  • Extensive contamination controls: blanks for KOH, formalin, plasticware spectra; within-sample CV ≈ 25\%.

Results – Total MNP Concentrations

  • 2024 median concentrations (µg g⁻¹):
    • Liver 433; Kidney 404; Brain 4917.
  • 2016 medians:
    • Liver \approx 300 (exact 2016 value in paper figure); Kidney similar; Brain 3345.
  • Brain ≫ Liver/Kidney (two-way ANOVA P < 0.0001).
  • Liver + Brain 2024 > 2016 (Mann-Whitney / post-hoc; brain P = 0.01).
  • East-coast brains older (1997–2013): median 1254\,\mu\text{g g}^{-1} — lower than NM 2016 & 2024.

Polymer Composition Patterns

  • Overall mass dominated by polyethylene (PE).
    • Brain average PE share \approx 75\%, significantly higher than liver/kidney (P < 0.0001).
  • Secondary contributors: PP, PVC, SBR.
  • Temporal increase (2016→2024) significant for PE, PP, PVC, SBR in liver & brain.
  • FTIR confirmed PE dominance; other polymers less consistently detected by FTIR due to size biases.

Dementia Cohort Findings

  • Dementia brains median total plastics 26076\,\mu\text{g g}^{-1} → \sim 5-fold higher than normal 2024 brains.
  • Light microscopy: dense refractile inclusions along cerebrovascular walls & within immune-cell-rich regions.
  • Authors caution: association only; dementia-related atrophy, BBB breakdown & impaired clearance could facilitate accumulation.

Microscopy & Morphology Observations

  • Liver & Kidney:
    • Polarization microscopy: widespread refractile inclusions; rod-like 1\text{–}5\,\mu\text{m} particles within lipid droplets, glomeruli, tubules.
    • TEM of isolated pellets: shard-like flakes < 0.4\,\mu\text{m} length, < 40\,\text{nm} width.
    • SEM-EDS: carbon-rich, non-mineral.
  • Brain:
    • Light microscopy: few >1 µm inclusions; abundant sub-µm particles.
    • TEM: innumerable shards 100\text{–}200\,\text{nm} long.
    • SEM-EDS confirms carbon composition.
  • Postulated aggregation of nanoplastics in liver/kidney vs dispersed state in brain.

Temporal Trend Analysis

  • Simple linear regression on normal brains (1997 → 2024):
    • Total plastics, PE, PP, PVC, SBR: positive slope; overall R^2 = 0.3982; P < 0.0001.
  • No correlation between age of decedent and brain MNP load (P = 0.87) ⇒ lifetime accumulation not primary driver; environmental rise more likely.
  • Approx. 50\% increase in brain MNP mass over past 8 years.

Mechanistic Hypotheses

  • Uptake routes: ingestion with lipids → chylomicron transport, clathrin-dependent endocytosis, macropinocytosis (citing \textit{Daphnia magna} data).
  • BBB passage mechanism unclear; nanoplastic size (<200 nm) may facilitate translocation.
  • Possible dynamic equilibrium between exposure, uptake, and clearance; environmental concentrations set internal ceiling.

Analytical Caveats & Limitations

  • Residual biomatrix in pellets may cause spectral interference; lipids can mimic PE pyrolysates.
  • KOH digestion removed \approx 99.4\% of liver/kidney mass, 91.8\% of brain; brain pellets larger (≈41 mg) → proportional to measured MNP mass.
  • Oxidative aging of environmental polymers produces shorter pyrolysis fragments → potential under-quantification vs pristine standards.
  • Ultracentrifugation may lose smallest nanoparticles ⇒ further underestimation.
  • Single-site sampling per organ per donor ⇒ intra-organ heterogeneity not captured.
  • Formalin storage times differed (2016 samples stored 84–96 mo, 2024 2–4 mo); yet 2024 showed higher loads, arguing against storage contamination.

Statistical Methods Summary

  • Two-way ANOVA for organ × year factors; post-hoc multiple comparisons.
  • Mann-Whitney tests (two-sided) for pairwise year comparisons within organ.
  • Student’s t test to compare UNM vs OSU replicate brains (P = 0.49).
  • Multiple regression incorporating demographic covariates (details in Supplementary Tables 8–10).
  • Regression of concentration vs year (normal brains) for trend analysis.
  • Within-sample coefficient of variation on Py-GC/MS ≈ 25\% across both labs.

Broader Context & Implications

  • Brain burdens 7\text{–}30× higher than liver/kidney; dementia brains even higher.
  • Findings align with recent reports of MNPs in carotid plaques linked to cardiovascular risk.
  • Predominance of PE nanoparticles (nanoscale shards/flakes) matches degradation products predicted for environmental weathering.
  • Highlights urgent need to:
    • Elucidate exposure pathways & BBB transport mechanisms.
    • Determine clearance kinetics & potential neurotoxic effects.
    • Standardize analytical protocols for clinical monitoring.
    • Investigate causal links to neurodegenerative diseases.
  • Exponential rise in global MNP production & environmental concentration (\sim 400\,\text{Mt yr}^{-1} plastic produced; ref. 20) implies continued increase in human tissue burdens without intervention.

Ethical & Practical Considerations

  • Autopsy studies classified as “not human research” by IRB; limited identifiers preserved donor anonymity.
  • Ethical imperative to assess public-health impact of ubiquitous pollutant.
  • Potential policy relevance for plastic production, waste management, and consumer exposure mitigation.

Key Numerical Highlights

  • Size spectrum studied: 1\,\text{nm}\text{–}500\,\mu\text{m}.
  • Peak brain concentration (dementia case): >50,000\,\mu\text{g g}^{-1} in outliers.
  • Typical shard dimensions (TEM): length <200\,\text{nm}, width <40\,\text{nm}.
  • Reduction of biological mass post-digestion: liver/kidney \approx 99.4\%, brain \approx 91.8\%.

Connections to Prior Literature

  • First detection of microplastics in human blood (Leslie et al., 2022) & arteries (Liu et al., 2024) set precedent for internal polymer quantification.
  • Complement to studies showing exacerbated disease in animal models exposed to high MNP doses.
  • Extends Plastics & Human Health Commission (Landrigan et al., 2023) call for multidisciplinary research.

Suggested Study Questions

  • Why might polyethylene accumulate preferentially in the brain compared to other polymers?
  • Discuss potential routes by which nanoplastics could cross the blood–brain barrier.
  • How might oxidative weathering of plastics complicate Py-GC/MS quantification?
  • Evaluate strengths and weaknesses of autopsy-based exposure assessment versus live biomonitoring.
  • Propose experimental designs to test causality between brain MNP load and neurodegenerative disease.