New insights into the toxicity of mineral fibres: A combined in situ synchrotron μ-XRD and HR-TEM study of chrysotile, crocidolite, and erionite fibres found in the tissues of Sprague-Dawley rats

Our plain-English summary of New insights into the toxicity of mineral fibres: A combined in situ synchrotron μ-XRD and HR-TEM study of chrysotile, crocidolite, and erionite fibres found in the tissues of Sprague-Dawley rats, by A.F. Gualtieri, N. Bursi Gandolfi, S. Pollastri, et al (Toxicology Letters, 2017).

The study

• Authors: A.F. Gualtieri, N. Bursi Gandolfi, S. Pollastri, M. Burghammer, E. Tibaldi, F. Belpoggi, K. Pollok, F. Langenhorst, R. Vigliaturo, G. Dražić
• Journal: Toxicology Letters
• Published: 2017;274:20-30
• Cited by: 9 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Along the line of the recent research topic aimed at understanding the in vivo activity of mineral fibres and their mechanisms of toxicity, this work describes the morpho-chemical characteristics of the mineral fibres found in the tissues of Sprague-Dawley rats subjected to intraperitoneal/intrapleural injection of UICC chrysotile, UICC crocidolite and erionite-Na from Nevada (USA). The fibres are studied with in situ synchrotron powder diffraction and high resolution transmission electron microscopy to improve our understanding of the mechanisms of toxicity of these mineral fibres. In contact with the tissues of the rats, chrysotile fibres are prone to dissolve, with leaching of Mg and production of a silica rich relict. On the other hand, crocidolite and erionite-Na fibres are stable even for very long contact times within the tissues of the rats, showing just a thin dissolution amorphous halo. These findings support the model of a lower biopersistence of chrysotile with respect to crocidolite and erionite-Na but the formation of a silica-rich fibrous residue after the pseudo-amorphization of chrysotile may justify a higher cytotoxic potential and intense inflammatory activity of chrysotile in the short term in contact with the lung tissues.

Copyright © 2017 Elsevier B.V. All rights reserved.

In plain English

This study examined fibres from tissues of Sprague-Dawley rats used in earlier tests at the Ramazzini cancer research institute near Bologna, Italy. Those rats had received a single 25 mg injection of UICC chrysotile, UICC crocidolite, or fibrous erionite-Na from Jersey, Nevada. The rats lived until natural death, which marked how long fibres stayed in the body. The authors studied nine tissue samples (38 to 104 weeks of contact) by synchrotron X-ray diffraction, and examined fibres by high-resolution electron microscopy. Chrysotile fibres became less crystalline over time, formed an amorphous outer layer, and some broke down into silica-rich particles. Some still-crystalline chrysotile fibres remained at 93 weeks. Crocidolite and erionite-Na fibres largely kept their crystallinity. Crocidolite showed a thin silica-rich amorphous layer and minor changes in its sodium-containing crystal sites; erionite-Na's silica-rich layer was thicker. The authors concluded that chrysotile reacts much faster than crocidolite and erionite. They also concluded that the silica-rich chrysotile residue may possibly cause intense short-term inflammation; confirming tests were in progress. They noted that injected fibre burdens may be excessive compared with inhalation.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory mineralogical study of fibres found in tissue samples from an earlier animal injection experiment, using synchrotron micro X-ray diffraction and high-resolution transmission electron microscopy
• Who was studied: Tissue samples (pleural tissues) from Sprague-Dawley rats used in past in vivo tests at the Ramazzini Cancer Research Institute, Bentivoglio, Bologna, Italy. For the synchrotron X-ray diffraction work, the authors studied nine tissue samples: crocidolite after 38, 59 and 80 weeks of contact time; chrysotile after 71, 80 and 93 weeks; erionite-Na after 63, 79 and 104 weeks. They also studied raw fibre samples for comparison. Fibres were examined separately by electron microscopy after being separated from the tissue.
• Exposure: UICC standard chrysotile, UICC standard crocidolite, and fibrous erionite-Na from Jersey (Nevada, USA). Each fibre was given as a single 25 mg injection in 1 ml of water (intraperitoneal/intrapleural) to groups of 40 rats (20 males and 20 females) aged 6 to 8 weeks. Contact time was the time from injection to spontaneous death.
• Main results:
  • Chrysotile: the X-ray diffraction signal (the 001 reflection) decreased with contact time (raw, 71, 80 and 93 weeks). The authors read this as a statistical decrease in fibre crystallinity.
  • Chrysotile: some crystalline chrysotile fibres were still present at 93 weeks.
  • Chrysotile: electron microscopy showed an outer layer of amorphous (non-crystalline) material on fibrils after 71 weeks in the tissue.
  • Chrysotile: after prolonged contact, the final product of dissolution was amorphous silica-rich nanoparticles. These images were from 80 weeks of contact time.
  • Chrysotile: breakdown was uneven. Even at the longest contact times, intact crystalline fibres coexisted with partly decomposed and fully dissolved fibres.
  • Crocidolite: unlike chrysotile, the diffraction signal did not decrease with contact time (0, 38, 59 and 80 weeks).
  • Crocidolite: one fibre examined after 80 weeks had a silicon-rich amorphous surface layer with a mean thickness of 15.24 nm.
  • Crocidolite: a highly crystalline fibre showed a thin amorphous layer of 1 to 2 nm.
  • Crocidolite: a minor migration of sodium atoms between two sites in the crystal structure was observed with contact time.
  • Erionite-Na: the diffraction signal did not decrease with contact time (0, 63, 79 and 104 weeks), indicating preserved crystallinity.
  • Erionite-Na: after 104 weeks, the fibres examined had an amorphous, largely silica outer region extending from 0 to 80 nm. Crystalline material extended from 80 to 320 nm.
  • Erionite-Na: the silica-rich amorphous layer around the fibres was thicker than the layer seen on crocidolite.
  • Erionite-Na: chemical analysis showed a loss of extra-framework cations after 104 weeks.
• The authors' conclusion: The authors concluded, together with literature data, that chrysotile reacts much faster than crocidolite and erionite, which supports a lower biopersistence of chrysotile. They also concluded that the silica-rich fibrous residue left after chrysotile breaks down may prompt production of hydroxyl radicals together with release of active surface iron, possibly resulting in intense inflammatory activity in the short term.
• Limitations the authors state:
  • The diffraction data for chrysotile were of insufficient quality for structure refinement, so only qualitative or semi-quantitative indications were given.
  • Single-crystal data could not be collected on individual fibres.
  • Fibres may have clumped together during instillation, which could partly explain their persistence. The authors note this may be a difference from humans, because such clumps would be unlikely to reach these spaces through normal inhalation.
  • Fibre burdens from injection may be excessive compared with inhalation in rats and in humans (fibre overload).
  • The proposed toxicity model for the chrysotile dissolution product was not yet confirmed; tests to confirm it were in progress.

Topics

Medical terms in this study, explained in our Lexicon: Chrysotile, Crocidolite, erionite, UICC, Transmission Electron Microscopy (TEM), biopersistence.

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