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==Overview==
==Overview==
The toxicity of crystalline silica results from the ability of crystalline silica surfaces to interact with aqueous media, to generate [[oxygen radicals]], and to injure target pulmonary cells such as alveolar [[macrophages]]. Generation of inflammatory [[cytokines]] (eg, interleukin-1 and tumor necrosis factor beta) by target cells results in cytokine networking between inflammatory cells and resident pulmonary cells, which in turn leads to inflammation and fibrosis.
==Pathophysiology==
==Pathophysiology==
Silica (silicon dioxide) is the most abundant mineral on earth. Silica exists in crystalline and amorphous forms. Crystalline silica (quartz, cristobalite, and tridymite) is associated with a spectrum of pulmonary diseases. Amorphous forms, including vitreous silica and diatomite (formed from skeletons of prehistoric marine organisms), are relatively less toxic after inhalation [18].
Quartz is the most abundant form of crystalline silica and is a major component of rocks including granite, slate, and sandstone. Granite contains about 30 percent free silica, slate about 40 percent, and sandstone is almost pure silica [19]. Cristobalite and tridymite occur naturally in lava and are formed when quartz or amorphous silica is subjected to very high temperatures.
The toxicity of crystalline silica appears to result from the ability of crystalline silica surfaces to interact with aqueous media, to generate oxygen radicals, and to injure target pulmonary cells such as alveolar macrophages. Resultant generation of inflammatory cytokines (eg, interleukin-1 and tumor necrosis factor beta) by target cells lead to cytokine networking between inflammatory cells and resident pulmonary cells, resulting in inflammation and fibrosis [20].
"Free" crystalline silica is unbound to other minerals. "Combined" forms of silica, called silicates, are compounds in which silica is bound to other minerals. Examples of silicates used in industry include asbestos (hydrated magnesium silicate), talc (Mg3Si4O10(OH)2), and kaolinite (Al2Si2O5(OH)4), a major component of kaolin (china clay) [21]. The pulmonary effects of asbestos inhalation are substantial, and are discussed separately
Silica probably exerts it effects on the macrophages that ingest it and alter their function rather than disrupting it. The 
stimulated macrophage appears to secrete mediator substances such as Interleukin-1. The macrophages has been implicated to be the major cause of fibrosis  that accompanies silicosis.
When small silica dust particles are inhaled, they can embed themselves deeply into the tiny alveolar sacs and ducts in the lungs, where oxygen and carbon dioxide gases are exchanged. There, the lungs cannot clear out the dust by mucous or coughing.
When fine particles of silica dust  are deposited in the lungs, [[macrophage]]s that ingest the dust particles will set off an [[inflammation]] response by releasing tumor necrosis factors, [[interleukin-1]], [[leukotriene B4]] and other [[cytokines]].  In turn, these stimulate [[fibroblast]]s to proliferate and produce collagen around the silica particle, thus resulting in [[fibrosis]] and the formation of the nodular lesions.


Furthermore, the surface of silicon dust can generate silicon-based radicals that lead to the production of [[hydroxyl]] and oxygen radicals, as well as [[hydrogen peroxide]], which can inflict damage to the surrounding cells.
=== Pathogenesis ===
*The toxicity of crystalline silica appears to result from the ability of crystalline silica surfaces to interact with aqueous media, to generate [[oxygen radicals]], and to injure target pulmonary cells such as alveolar macrophages.
* Generation of inflammatory [[cytokines]] (eg, interleukin-1 and tumor necrosis factor beta) by target cells results in cytokine networking between inflammatory cells and resident pulmonary cells, which in turn leads to inflammation and fibrosis.<ref name="pmid15699791">{{cite journal| author=Rimal B, Greenberg AK, Rom WN| title=Basic pathogenetic mechanisms in silicosis: current understanding. | journal=Curr Opin Pulm Med | year= 2005 | volume= 11 | issue= 2 | pages= 169-73 | pmid=15699791 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=15699791  }} </ref>
* The alveolar macrophages are implicated as the major cell type in fibrogenesis<ref name="pmid7978983">{{cite journal| author=Oberdörster G| title=Macrophage-associated responses to chrysotile. | journal=Ann Occup Hyg | year= 1994 | volume= 38 | issue= 4 | pages= 601-15, 421-2 | pmid=7978983 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=7978983 ; }} </ref>, but other immune cells, namely neutrophils<ref name="pmid7677184">{{cite journal| author=Quinlan TR, BéruBé KA, Marsh JP, Janssen YM, Taishi P, Leslie KO et al.| title=Patterns of inflammation, cell proliferation, and related gene expression in lung after inhalation of chrysotile asbestos. | journal=Am J Pathol | year= 1995 | volume= 147 | issue= 3 | pages= 728-39 | pmid=7677184 | doi= | pmc=PMC1870980 | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=7677184 ; }} </ref>, T-lymphocytes, and mast cells are also involved.
* Following the interaction between effector immune cells (such as alveolar macrophage) and target tissue (such as bronchiolar/alveolar epithelial cells, [[fibroblasts]]), the progression of the disease is poorly understand.
** Injury to the alveolar type I epithelial cell is regarded as an early event in [[fibrogenesis]] followed by [[hyperplasia]] and [[hypertrophy]]<ref name="pmid7547443">{{cite journal| author=Lesur O, Bouhadiba T, Melloni B, Cantin A, Whitsett JA, Bégin R| title=Alterations of surfactant lipid turnover in silicosis: evidence of a role for surfactant-associated protein A (SP-A). | journal=Int J Exp Pathol | year= 1995 | volume= 76 | issue= 4 | pages= 287-98 | pmid=7547443 | doi= | pmc=PMC1997178 | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=7547443 ; }} </ref> of type II epithelial cells.
** Silica-induced cell hyperproliferation of mesenchymal cells is also a hallmark of the fibrotic lesion.
** Proliferation may occur intially at sites of accumulation of inhaled minerals, but later at distal sites where particles or fibers are translocated over time.
** Alternatively, mitogenic cytokines may mediate signaling events, leading to cell replication at sites physically remote from fibers.
** The initiation of proliferation in epithelial cells and [[fibroblasts]] by [[silica]] may occur following the upregulation of the early response proto-oncogenes C-FOS, C-JUN, and C-MYC.<ref name="pmid7946382">{{cite journal| author=Janssen YM, Heintz NH, Marsh JP, Borm PJ, Mossman BT| title=Induction of c-fos and c-jun proto-oncogenes in target cells of the lung and pleura by carcinogenic fibers. | journal=Am J Respir Cell Mol Biol | year= 1994 | volume= 11 | issue= 5 | pages= 522-30 | pmid=7946382 | doi=10.1165/ajrcmb.11.5.7946382 | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=7946382 ; }} </ref>
** Increased expression of early response genes and protein products is also linked to the development of [[apoptosis]]<ref name="pmid8679218">{{cite journal| author=BéruBé KA, Quinlan TR, Fung H, Magae J, Vacek P, Taatjes DJ et al.| title=Apoptosis is observed in mesothelial cells after exposure to crocidolite asbestos. | journal=Am J Respir Cell Mol Biol | year= 1996 | volume= 15 | issue= 1 | pages= 141-7 | pmid=8679218 | doi=10.1165/ajrcmb.15.1.8679218 | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=8679218 ; }} </ref><ref name="pmid9603153">{{cite journal| author=Mossman BT, Churg A| title=Mechanisms in the pathogenesis of asbestosis and silicosis. | journal=Am J Respir Crit Care Med | year= 1998 | volume= 157 | issue= 5 Pt 1 | pages= 1666-80 | pmid=9603153 | doi=10.1164/ajrccm.157.5.9707141 | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=9603153  }} </ref>


Characteristic lung tissue pathology in nodular silicosis consists of fibrotic nodules with concentric "onion-skinned" arrangement of [[collagen]] fibers, central hyalinization, and a cellular peripheral zone, with lightly birefringent particles seen under polarized light. In acute silicosis, microscopic pathology shows a periodic acid-Schiff positive alveolar exudate (alveolar lipoproteinosis) and a cellular infiltrate of the alveolar walls.
=== Low Intensity Exposure vs. High Intensity Exposure ===
* Lower intensity exposures to silica evoke reversible inflammatory changes characterized by focal aggregations of mineral-laden alveolar [[Macrophages|macrophages.]]<ref name="pmid8265248">{{cite journal| author=Velan GM, Kumar RK, Cohen DD| title=Pulmonary inflammation and fibrosis following subacute inhalational exposure to silica: determinants of progression. | journal=Pathology | year= 1993 | volume= 25 | issue= 3 | pages= 282-90 | pmid=8265248 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=8265248  }} </ref> 
* In contrast, higher exposures elicit intense and protracted inflammatory changes, cell proliferation in various compartments of the lung, and excessive deposition of collagen and other extracellular matrix components by mesenchymal cells.


==References==
==References==
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[[Category:Pulmonology]]
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Latest revision as of 15:26, 8 June 2016

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Aparna Vuppala, M.B.B.S. [2]

Overview

The toxicity of crystalline silica results from the ability of crystalline silica surfaces to interact with aqueous media, to generate oxygen radicals, and to injure target pulmonary cells such as alveolar macrophages. Generation of inflammatory cytokines (eg, interleukin-1 and tumor necrosis factor beta) by target cells results in cytokine networking between inflammatory cells and resident pulmonary cells, which in turn leads to inflammation and fibrosis.

Pathophysiology

Pathogenesis

  • The toxicity of crystalline silica appears to result from the ability of crystalline silica surfaces to interact with aqueous media, to generate oxygen radicals, and to injure target pulmonary cells such as alveolar macrophages.
  • Generation of inflammatory cytokines (eg, interleukin-1 and tumor necrosis factor beta) by target cells results in cytokine networking between inflammatory cells and resident pulmonary cells, which in turn leads to inflammation and fibrosis.[1]
  • The alveolar macrophages are implicated as the major cell type in fibrogenesis[2], but other immune cells, namely neutrophils[3], T-lymphocytes, and mast cells are also involved.
  • Following the interaction between effector immune cells (such as alveolar macrophage) and target tissue (such as bronchiolar/alveolar epithelial cells, fibroblasts), the progression of the disease is poorly understand.
    • Injury to the alveolar type I epithelial cell is regarded as an early event in fibrogenesis followed by hyperplasia and hypertrophy[4] of type II epithelial cells.
    • Silica-induced cell hyperproliferation of mesenchymal cells is also a hallmark of the fibrotic lesion.
    • Proliferation may occur intially at sites of accumulation of inhaled minerals, but later at distal sites where particles or fibers are translocated over time.
    • Alternatively, mitogenic cytokines may mediate signaling events, leading to cell replication at sites physically remote from fibers.
    • The initiation of proliferation in epithelial cells and fibroblasts by silica may occur following the upregulation of the early response proto-oncogenes C-FOS, C-JUN, and C-MYC.[5]
    • Increased expression of early response genes and protein products is also linked to the development of apoptosis[6][7]

Low Intensity Exposure vs. High Intensity Exposure

  • Lower intensity exposures to silica evoke reversible inflammatory changes characterized by focal aggregations of mineral-laden alveolar macrophages.[8]
  • In contrast, higher exposures elicit intense and protracted inflammatory changes, cell proliferation in various compartments of the lung, and excessive deposition of collagen and other extracellular matrix components by mesenchymal cells.

References

  1. Rimal B, Greenberg AK, Rom WN (2005). "Basic pathogenetic mechanisms in silicosis: current understanding". Curr Opin Pulm Med. 11 (2): 169–73. PMID 15699791.
  2. Oberdörster G (1994). ; "Macrophage-associated responses to chrysotile" Check |url= value (help). Ann Occup Hyg. 38 (4): 601–15, 421–2. PMID 7978983.
  3. Quinlan TR, BéruBé KA, Marsh JP, Janssen YM, Taishi P, Leslie KO; et al. (1995). ; "Patterns of inflammation, cell proliferation, and related gene expression in lung after inhalation of chrysotile asbestos" Check |url= value (help). Am J Pathol. 147 (3): 728–39. PMC 1870980. PMID 7677184.
  4. Lesur O, Bouhadiba T, Melloni B, Cantin A, Whitsett JA, Bégin R (1995). ; "Alterations of surfactant lipid turnover in silicosis: evidence of a role for surfactant-associated protein A (SP-A)" Check |url= value (help). Int J Exp Pathol. 76 (4): 287–98. PMC 1997178. PMID 7547443.
  5. Janssen YM, Heintz NH, Marsh JP, Borm PJ, Mossman BT (1994). ; "Induction of c-fos and c-jun proto-oncogenes in target cells of the lung and pleura by carcinogenic fibers" Check |url= value (help). Am J Respir Cell Mol Biol. 11 (5): 522–30. doi:10.1165/ajrcmb.11.5.7946382. PMID 7946382.
  6. BéruBé KA, Quinlan TR, Fung H, Magae J, Vacek P, Taatjes DJ; et al. (1996). ; "Apoptosis is observed in mesothelial cells after exposure to crocidolite asbestos" Check |url= value (help). Am J Respir Cell Mol Biol. 15 (1): 141–7. doi:10.1165/ajrcmb.15.1.8679218. PMID 8679218.
  7. Mossman BT, Churg A (1998). "Mechanisms in the pathogenesis of asbestosis and silicosis". Am J Respir Crit Care Med. 157 (5 Pt 1): 1666–80. doi:10.1164/ajrccm.157.5.9707141. PMID 9603153.
  8. Velan GM, Kumar RK, Cohen DD (1993). "Pulmonary inflammation and fibrosis following subacute inhalational exposure to silica: determinants of progression". Pathology. 25 (3): 282–90. PMID 8265248.


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