Intra-alveolar Fibrosis Research Articles

Pathologic and ultrastructural findings in sheep in Morocco with naturally occuring jaagsiekte (sheep pulmonary adenomatosis)

Jaagsiekte was studied in the lungs of 85 sheep of Sardi and mixed breeds ranging from 7 months to 8 years of age. Lungs were examined grossly, histologically and, in five cases, by transmission electron microscopy. Lesions in lungs with jaagsiekte were described: they consisted of multiple focal to coalescent neoplastic nodules. Microscopic architecture of the neoplasms was similar in all cases, and lesions were seen as foci of cells arranged in glandular-like patterns (acinar and/or papillary). Intraalveolar fibrosis was prominent in many cases, specially in chronic ones, and alveolar macrophages were consistently found within alveolar spaces. In some cases (38/85), nodules or masses of myxomatous-like tissue were seen in the vicinity of adenomatous areas. Associated verminous pneumonia, hydatid cysts, and secondary bacterial bronchopneumonia were frequently encountered. Ultrastructurally, the proliferating neoplastic cells appeared to result from the transformation of Type II pneumocytes. Neither metastatic lesions nor viral inclusions were observed during this investigation. Sheep in the present study were not tested for antibodies to ovine lentivirus, maedi/visna. This descriptive study provides a morphological background necessary for routine differentiation and indicates the necessity of initiating studies on viral etiology and pathogenesis of jaagsiekte in Morocco.

Evolution of three patterns of intra-alveolar fibrosis produced by bleomycin in rats.

In pulmonary fibrosis, it is known that fibrotic changes develop in the intra-alveolar spaces and that intra-alveolar fibrosis can be classified into three patterns, namely intra-alveolar buds, mural incorporation and obliterative changes. In order to clarify the evolution of intra-alveolar fibrosis, immunohistochemical studies of extracellular matrix proteins and electron microscopic observations were made of the lungs of rats given a single intratracheal instillation of bleomycin. All three patterns of fibrosis developed in this model. Intra-alveolar buds changed into globular lesions with dense collagen deposition, the surface of which was covered by alveolar epithelium. Electron microscopy revealed that the buds often contained spiraling collagen fibrils and numerous microfibrils, but not mature elastic fibres, beneath the regenerating epithelial lining cells; the epithelial basement membranes were discontinuous. In contrast, mural incorporation and obliterative changes were associated with alveolar structural remodeling. Electron microscopically, these lesions had bundles of normal collagen fibrils, small elastic fibers, and continuous epithelial basement membranes. These results indicate that: (i) intra-alveolar buds, that become intra-alveolar collagen globules, with an unusual extracellular matrix, do not contribute to alveolar structural remodelling; and (ii) areas of mural incorporation and obliterative changes have the usual type of extracellular matrix and are essential for alveolar structural remodelling.

Alveolar fibrosis and capillary alteration in experimental pulmonary silicosis in rats.

To analyze the evolution of fibrotic and vascular changes in pulmonary silicosis, ultrastructural and immunohistochemical studies were made of the lungs of rats given a single intratracheal injection of silica particles. Early lesions were characterized by accumulations of macrophages and neutrophils in alveolar lumina and interstitium and by damage to alveolar capillaries and epithelial cells. The intraluminal masses of inflammatory cells developed into granulomas and became associated with myofibroblasts that migrated from the interstitium through the damaged epithelial lining. Type II epithelial cells and bronchiolar cuboidal cells proliferated rapidly to line the intraluminal granulomas, incorporating them into the interstitium. This process mediated the transition from intraalveolar fibrosis to interstitial fibrosis. Vascular damage was repaired by proliferation and migration of endothelial cells. Some endothelial cells in alveolar capillaries expressed Factor VIII-related antigen at 2 wk after silica infusion. In normal animals, this feature was present in peribronchiolar but not in alveolar capillaries. Two patterns of endothelial cell migration were shown by staining for proliferating-cell nuclear antigen. The first pattern was characterized by endothelial cells that extended their cytoplasm over preexisting, denuded basement membranes and replaced necrotic cells in alveolar capillaries. At 4 mo after injury, some of these cells had developed fenestrations. The second pattern consisted of budlike sproutings that developed only in peribronchiolar connective tissue. These observations indicate that peribronchiolar vessels are sources for renewal of alveolar capillary endothelium as well as for neovascularization.

Significance of early intra-alveolar fibrotic lesions and integrin expression in lung biopsy specimens from patients with idiopathic pulmonary fibrosis

To study the pulmonary structural remodeling in idiopathic pulmonary fibrosis (IPF), ultrastructural, immunohistochemical, and light microscopic morphometric observations were made on 11 pulmonary biopsy specimens from patients with IPF. The morphometric study was done using sequentially cut tissue sections stained for keratin-alcian blue periodic acid-Schiff (PAS), fibronectin, and type IV collagen-alcian blue PAS. Most of the early fibrotic lesions, which were alcian blue-and fibronectin-positive, were intra-alveolar in location. Intra-alveolar fibrosis is considered to be essential for the fusion of alveolar walls in IPF. A strong reaction for integrin α5β1 and vinculin was found in epithelial cells and mesenchymal cells in areas of intra-alveolar fibrosis. These findings show that these cells are active in adhesion to fibronectin in areas of early intra-alveolar fibrosis. Some of the epithelial cells, including cytoplasmic hyaline-laden cells, showed evidence of inadequate adhesion to the extracellular matrix, and this may constitute one of the mechanisms of progression of fibrosis in IPF.

Expression of transforming growth factor-alpha and epidermal growth factor receptor is increased following bleomycin-induced lung injury in rats.

To investigate the potential role of transforming growth factor-alpha (TGF-alpha) and the epidermal growth factor receptor (EGF-R) in the fibroproliferative response to acute lung injury, we determined lung steady-state TGF-alpha and EGF-R mRNA levels, TGF-alpha protein levels, and the distribution of TGF-alpha and EGF-R immunoreactive protein of bleomycin-injured and control rat lungs. At 2 and 4 days after a single intratracheal injection of bleomycin, TGF-alpha mRNA levels increased to 159% and 184% of control values, respectively. EGF-R mRNA levels increased to 163%, 314%, and 170% of control values at 1, 7, and 14 days after bleomycin instillation. TGF-alpha protein levels in whole lung extracts increased to 230% of control values at 4 days after bleomycin administration. TGF-alpha and EGF-R immunoreactivity was detected in macrophages, alveolar septal cells, and airway epithelium of control and bleomycin-injured animals with an apparent increase in the intensity and number of specifically immunostained cells following lung injury. TGF-alpha and EGF-R immunoreactive proteins were detected in foci of cellular proliferation and in areas of intraalveolar fibrosis. We conclude that TGF-alpha and the EGF-R are present in normal and bleomycin-injured rat lung and that the expression of this growth factor and its receptor are up-regulated following lung injury. These results suggest that increased expression of TGF-alpha and the EGF-R may be an important mechanism that modulates the fibroproliferative response to acute lung injury.

Acute Lung Injury: A Transgenic Murine Model of Intra-alveolar Fibrosis

Acute Lung Injury: A Transgenic Murine Model of Intra-alveolar Fibrosis

Four Markers of Collagen Metabolism as Possible Indicators of Disease in the Adult Respiratory Distress Syndrome

During the adult respiratory distress syndrome (ARDS), an irreversible fibrotic process can occur extremely rapidly. To establish indices of ARDS in pneumonia as well as the severity of the lung fibrosis, we have undertaken for the first time a study of four markers of collagen metabolism obtained from both bronchoalveolar lavage fluid (BALF) and serum: Type I (CI), Type III (CIII), N-terminal peptide of Type III procollagen (PIIINP), and galactosylhydroxylysylglucosyltransferase activity (GGT). We studied 61 patients (13 coma controls, 29 with pneumonia, and 19 with ARDS). In BALF, the average values of CI, CIII, PIIINP, and GGT were significantly higher in ARDS than in the control patients. The values for patients with pneumonia, although increased, were significantly lower than those in ARDS for CI, CIII, and PIIINP. In serum, the mean CI and PIIINP were significantly increased in pneumonia and ARDS, but the mean CIII was significantly increased only in ARDS compared with the control group. Significant positive linear correlations were observed for ARDS between CI and CIII or PIIINP and CIII in BALF and serum. Such correlations were observed for pneumonia only in serum. Molecular mass determinations demonstrated that CI- and CIII-related antigens in BALF were essentially intact triple helices of collagens or procollagens. Among patients with histologically defined interstitial fibrosis, the level of PIIINP in BALF was significantly higher for those with an additional intraalveolar fibrosis. In conclusion, measurements of these collagen markers may be useful for assessing disease activity and reflecting the flux of collagen molecules in the lung.

In Vitro Effects of Pentoxifylline on Smooth Muscle Cell Migration and Blood Monocyte Production of Chemotactic Activity for Smooth Muscle Cells: Potential Therapeutic Benefit in the Adult Respiratory Distress Syndrome

The adult respiratory distress syndrome (ARDS) is a severe lung condition characterized by an acute lung injury leading to a massive intra-alveolar fibrosis with rapid lung failure. ARDS intra-alveolar fibrosis results from the migration of mesenchymal cells (mainly smooth muscle cells [SMC]) into the alveoli through alveolar epithelial basement membrane gaps resulting from the injury. SMC migration is followed by their replication and production of extracellular matrix, which leads to fibrosis. Thus, any pharmacologic agent able to prevent SMC migration should prevent, at least in part, intra-alveolar fibrosis. SMC migration is thought to be due to the presence, in the alveolar spaces, of chemotactic factors for mesenchymal cells, such as fibronectin and platelet-derived growth factor (PDGF). The local presence of these chemotactic factors can be due to plasmatic leakage, platelet degranulation, and mononuclear phagocyte activation. Pentoxifylline is a methylxanthine interacting with the biology of several types of cells, including red blood cells, neutrophils, blood monocytes, and endothelial cells. Pentoxifylline prescription has been suggested in ARDS with respect to its activity on neutrophils, its inhibition of tumor necrosis factor-alpha (TNF) release by mononuclear phagocytes, and its prevention of TNF-induced lung injury. Since pentoxifylline can modulate the migration of several cell types, we hypothesized that it could interfere with mesenchymal cell migration. SMC migratory response was measured in vitro with modified Boyden chemotactic chambers in the presence of PDGF, fibronectin, "platelet extract," and activated blood monocyte supernatants. Pentoxifylline, at therapeutic levels, significantly reduced SMC migration in response to the presence of these chemotactic activities.(ABSTRACT TRUNCATED AT 250 WORDS)

Mesenchymal cells isolated after acute lung injury manifest an enhanced proliferative phenotype.

After acute lung injury, mesenchymal cells migrate into the alveolar airspace where they proliferate and deposit connective tissue macromolecules. Early in the disease process, inflammatory cell-derived trophic factors modulate these mesenchymal cell functions. However, in those patients who die, even as the inflammatory response abates, the fibroproliferative response continues, resulting in extensive intraalveolar fibrosis. We therefore hypothesized that lung mesenchymal cells obtained from individuals dying with acute alveolar fibrosis would manifest an enhanced proliferative capacity that was independent of persistent exogenous signals. To examine this hypothesis, the in vitro growth properties of mesenchymal cells prepared from patients dying with acute lung injury (n = 3) were analyzed in defined medium and compared with those of mesenchymal cells similarly prepared from patients dying with histologically normal lungs (n = 3). Isolates were characterized as mesenchymal cells by using morphological and immunohistochemical criteria. In accord with the hypothesis, mesenchymal cells isolated from lung-injured patients doubled within 3 d in the complete absence of exogenous peptide growth factors, reaching a saturation density of approximately 15 x 10(3) cells/cm2. As expected, lung mesenchymal cells from normal individuals failed to significantly increase in number. Consistent with this proliferative phenotype, the immediate early cell division cycle genes c-fos and c-jun were constitutively expressed in each cell strain prepared from injured lungs, but not in those from control lungs. The observed proliferative phenotype was stable through the fifth subcultivation of the cells. Despite these proliferative properties, three separate criteria indicated the mesenchymal cells from injured lungs were not transformed: normal karyotype; finite lifespan in vitro (9-10 subcultivations); and inability to disseminate in mice with severe combined immunodeficiency. These data support the hypothesis that mesenchymal cells manifest an enhanced proliferative state after acute lung injury.

Pulmonary vascular lesions in the adult respiratory distress syndrome caused by inhalation of zinc chloride smoke: A morphometric study

Two soldiers were fatally injured by accidental inhalation of zinc chloride (ZnCl2) from a smoke bomb. Although exposed to a relatively short but high smoke concentration, acute injury was minor and for 10 days the patients were clinically satisfactory. Unexpectedly, both then rapidly developed features typical of severe adult respiratory distress syndrome with pulmonary hypertension. Intubation and mechanical ventilation were instigated on day 15 (patient no. 1) and day 12 (patient no. 2) after the inhalation, but death followed at days 25 and 32, respectively. Lung vascular injury was assessed by angiography and morphometric techniques. The lungs showed extensive interstitial and intra-alveolar space fibrosis. Vessels showed a significant lumen reduction by contracture (that is, reduction in vessel external diameter) affecting preacinar and intraacinar arterial and venous segments, the extent of injury suggesting that hexite causes more severe venous injury than seen in other types of adult respiratory distress syndrome. In microvessels there was obliteration and widespread occlusion by endothelial cell proliferation and clot. No evidence of infection was identified during life or at autopsy. It is unclear whether the long lag time was due to the fact that the infection was not a complicating event or because steroids, administered prophylactically, had sufficed to delay, but not to prevent, the amplification of injury that seems responsible for the adult respiratory distress syndrome.

Augmentation of Functional Prostaglandin E Levels on the Respiratory Epithelial Surface by Aerosol Administration of Prostaglandin E

Prostaglandin E (PGE), a cyclooxygenase metabolite normally present in high concentrations in respiratory epithelial lining fluid (ELF), is capable of suppressing mesenchymal cell proliferation mediated by polypeptide-derived growth factors. Although PGE is normally abundant in respiratory ELF, PGE levels in ELF of individuals with idiopathic pulmonary fibrosis (IPF), a fibrotic lung disorder characterized by intraalveolar mesenchymal cell accumulation and fibrosis, were found to be 50% lower than normal (p less than 0.01): that is, a relative PGE "deficiency" in ELF may enhance intraalveolar mesenchymal cell proliferation in IPF. With this background, it is rational to consider augmenting PGE levels in ELF as a future therapy for IPF. Since systemic administration of PGE is associated with significant adverse effects, in vitro and experimental animal studies were carried out to evaluate whether aerosol PGE administration could augment ELF PGE levels. Greater than 50% of a solution of PGE1 could be placed in droplets less than 3 microns mass median aerodynamic diameter without loss of function. Aerosolization of PGE1 to sheep (n = 14) resulted in a marked augmentation of ELF PGE1 levels (preaerosol 20 +/- 7 nM, 30 min postaerosol 1,150 +/- 210 nM; p less than 0.0 to 0.1). ELF PGE1 levels remained elevated for up to 2 h (p less than 0.05 compared with baseline) and returned to baseline by 3 h (p greater than 0.2). Lung interstitial fluid (lymph) PGE1 levels increased slightly, but to levels far less than ELF levels (preaerosol 7 +/- 1 nM, 30 min postaerosol 13 +/- 2 nM; p less than 0.01), and plasma PGE1 levels did not change (p greater than 0.1).(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular Diversity of Basement Membrane Collagen: Elucidation of the Goodpasture's Epitope

Basement membranes (BMs) are specialized forms of ex­ tracellular matrix composed of collagens, noncollagenous glycoproteins such as laminin and entactin, and specific pro­ teoglycans. BMs delimit the various anatomic compartments of the lung, anchor epithelial and endothelial cells to the in­ terstitial matrix, and regulate the permeability properties of the alveolar wall. They are believed to provide a substratum for lung epithelial cell growth and differentiation and to play crucial roles in pulmonary morphogenesis. Integrity of the epithelial BM could also playa key role in determining the pattern of structural remodeling following lung injury. For example, disruption of the BM can allow interstitial connec­ tive tissue cells to migrate into areas of airspace exudate with resultant intra-alveolar fibrosis or bronchiolitis obliterans. Type IV collagen is an integral structural component of epithelial and endothelial BMs. It is synthesized by lung epi­ thelial, endothelial, smooth muscle, and perineural cells and by activated lung fibroblasts. The majority of type IV colla­ gen in most tissues is composed of heterotrimers of geneti­ cally distinct o l and a2 chains, i.e., [a1(IV)ha2. How­ ever, sequences corresponding to two less abundant chain types, a3 and a4(IV), have been identified in glomerular basement membrane (GBM) and other tissues. Although the molecular organization of the minor chains has not been fully elucidated, there is evidence for the formation of homopolymers, as well as heterotrimers with one or more of the major chain types. A fifth chain, a5(IV), is a compo­ nent of the GBM and has been assigned to the locus of X-linked Alport's familial nephritis (1-3). The complete amino acid sequences for the human and mouse o l and a2(IV) chains have been reported (4). Recent studies have also shown that the human a 1 and a2(IV) genes are closely linked on the long arm of chromosome 13, on op­ posite DNA strands with an overlapping promoter (5-7). However, the complete primary structure of the a3 and a4 chains and the chromosomal localization of the correspond­ ing genes have not yet been described.

Late BCNU lung: A light and ultrastructural study on the delayed effect of BCNU on the lung parenchyma

We describe eight patients who developed interstitial pulmonary fibrosis following BCNU (carmustine) therapy for cerebral tumours. The fibrosis presented 12-17 (mean 14) years after exposure to the drug. A distinctive pattern of pulmonary fibrosis with involvement of the apices and subpleural areas was seen in one patient dying of the disease. Light microscopy showed interstitial elastosis and intra-alveolar fibrosis which was often focal with an associated mild lymphoplasmacytic infiltrate, intra-alveolar oedema, macrophages, and some neutrophils. Ultrastructural studies showed electron lucency of type I pneumocytes, with breaks in the cytoplasmic membranes leaving a bare basement membrane. Degenerative change was also seen in endothelial cell cytoplasm along with lipofuscin deposition. While BCNU pulmonary fibrosis has been described up to 2 years after treatment, this complication so late after therapy, though rare, has important implications for the follow-up of patients receiving this drug.

Mechanisms of Alveolar Fibrosis Following Acute Lung Injury: Presence of Angiogenesis Bioactivity in the Lower Respiratory Tract

Survival after acute lung injury depends on prompt alveolar repair, a process frequently subverted by acute intraalveolar fibrosis in which exuberant growth of granulation tissue obliterates the alveolar air space. To gain insight into this process, we used immunohistochemical methods to examine the cellular populations present in the alveolus of lung specimens from 6 patients with acute lung injury. In addition to myofibroblasts, capillaries (factor VIII⊕) were present in the developing granulation tissue, indicating that angiogenesis was part of the pathologic process.

The acute toxicity of inhaled beryllium metal in rats

We exposed rats once by nose only for 50 min to a mean concentration of 800 μg/m 3 of beryllium metal (initial lung burden, 625 μg) to characterize the acute toxic effects within the lung. Histological changes within the lung and enzyme changes within bronchoalveolar lavage (BAL) fluid were evaluated at 3, 7, 10, 14, 31, 59, 115, and 171 days postexposure (dpe). Beryllium metal-exposed rats developed acute, necrotizing, hemorrhagic, exudative pneumonitis and intraalveolar fibrosis that peaked at 14 dpe. By 31 dpe, inflammatory lesions were replaced by minimal interstitial and intraalveolar fibrosis. Necrotizing inflammation was observed again at 59 dpe which progressed to chronic-active inflammation by 115 dpe. This inflammation worsened progressively, as did alveolar macrophage and epithelial hyperplasia, becoming severe at 171 dpe. Low numbers of diffusely distributed lymphocytes were also present but they were not associated with granulomas as is observed in beryllium-induced disease in man. Throughout the experiment, total numbers of cells were elevated within the BAL samples due primarily to increased numbers of neutrophils. Lymphocytes were not elevated in BAL samples collected from beryllium-exposed rats at any time after exposure. Lactate dehydrogenase (LDH), β-glucuronidase, and protein levels were elevated in BAL fluid from 3 through 14 dpe but returned to near normal levels by 31 dpe. LDH increased once again at 59 dpe and remained elevated at 171 dpe. β-Glucuronidase and protein levels were slightly, but not significantly, elevated from 31 through 171 dpe. Results indicate that inhalation of beryllium metal by rats results in severe, acute chemical pneumonitis that is followed by a quiescent period of minimal inflammation and mild fibrosis. Progressive, chronic-active, fibrosing pneumonitis is observed later. Chronic beryllium lung disease of man is an immunologically mediated granulomatous lung disease, whereas beryllium-induced lung lesions in rats appear to be due to direct chemical toxicity and foreign-body-type reactions.

Remodeling of the Lung in Bleomycin-induced Pulmonary Fibrosis in the Rat: An Immunohistochemical Study of Laminin, Type IV Collagen, and Fibronectin

Intratracheal injection of bleomycin in rats results in the development of patchy pulmonary fibrosis. We investigated events associated with remodeling of lung structure by light and electron microscopic immunolocalization of fibronectin, laminin, and type IV collagen. Animals were studied from 2 to 60 days after injection of bleomycin. In the acute phase of injury, staining of fibronectin was prominent in fibrinous exudates, whereas during healing it was mainly associated with the surface of fibroblasts. The early accumulation of fibronectin in alveolar exudates is probably the result of leakage of plasma. During the reparative phase, fibronectin may be synthesized locally since it is selectively associated with the fibroblast surface. Staining with antibodies to type IV collagen and laminin identified the basal lamina and also helped to define the tissue boundaries despite the presence of intense exudation. It highlighted two processes in the acutely injured lung that lead to abnormal lung architecture: collapse of alveoli and invasion of air spaces by fibroblasts. In some healed lesions at 20 and 60 days, the immunostaining still outlined atelectatic lung. Electron microscopy of these lesions showed collagenous synechiae between approximated alveolar walls. We suggest that alveolar collapse and intraalveolar fibrosis in areas of collapse play an important role in bleomycin-induced pulmonary fibrosis and probably other types of fibrosis. They readily explain the loss of lung volume and compliance characteristic of fibrotic lung.

Cell Proliferation during the Process of Bleomycin‐induced Pulmonary Fibrosis in Rats

To elucidate the relationship between cell proliferation and structural remodeling in pulmonary fibrosis, light and electron microscopy with immunohistochemistry for bromodeoxyuridine (BrdU) and morphometry for BrdU-positive cells were performed following a single intratracheal instillation of bleomycin in rats. The results showed that terminal bronchiolar epithelial cells, Type II alveolar epithelial cells and interstitial cells began to proliferate 2 days after the injury. Then each cell type showed a different style of proliferation. Interstitial cells which were located in the interstitium and migrated into intraalveolar spaces proliferated, then produced intra-alveolar fibrosis. Terminal bronchiolar epithelial cells proliferated rapidly and formed alveolar bronchiolization with squamous metaplasia in areas where alveoli were severely damaged and intraalveolar fibrosis was formed, and thereafter the proliferation ceased within 2 weeks. The degree of proliferation of type II alveolar epithelial cells and interstitial cells was rather slight, but continued constantly until the later stage. In addition, intraalveolar macrophages were BrdU-positive from an early stage. Endothelial cell proliferation was observed in small vessels and alveolar capillaries at 1 week after bleomycin instillation. Rapidly proliferating bronchiolar epithelial cells which formed alveolar bronchiolization with squamous metaplasia were important in preventing the progress of intraalveolar fibrosis, because the proliferation of type II alveolar epithelial cells was limited.

Evolution of metaplastic squamous cells of alveolar walls in pulmonary fibrosis produced by paraquat

Sequential histologic, ultrastructural, immunohistochemical and morphometric studies were made of the evolutional changes of metaplastic and regenerating alveolar epithelial cells in monkeys from 3 days to 8 weeks after paraquat administration. In the early proliferative phase, many alveoli were lined by single-layered and stratified squamous epithelium and bronchiolized epithelium (i.e., presumably derived from bronchi and bronchioles). The regenerating epithelial cells had well developed bundles of actin-like filaments, which were arranged parallel to the basal surfaces of the cells and were associated with zonulae adherentes; these cells also had intermediate filaments and some desmosomes, but lacked basement membranes, hemidesmosomes and anchoring fibrils. They covered either denuded, wavy and disrupted original epithelial basement membranes or areas of developing intraalveolar fibrosis. In zones of squamous epithelial cell metaplasia associated with intraalveolar fibrosis, fibronexus-like structures appeared to be responsible for the initial adhesion of the cells to the underlying connective tissue. In later phases, single-layered and stratified squamous epithelial cells disappeared, and only bronchiolized epithelial cells, with hemidesmosomes and anchoring fibrils on their basal surfaces, were found in fibrotic alveoli. Although bronchiolized and squamous metaplastic epithelial cells are generally thought to be formed as late events in pulmonary damage, such cells play an important role in early, temporary repair of damaged alveoli.

An immunohistochemical study of the fibrosing process in paraquat lung injury

Twenty-nine autopsy cases of paraquat-induced lung injury were studied by histological and immunohistochemical methods. Two stages of injury were identified. In the early stage, the alveolar epithelium degenerates but the epithelial basement membrane remains intact. In the late stage, the epithelial basement membrane is focally disrupted, the mesenchymal cells grow into the alveolar space, and intra-alveolar fibrosis appears. In spite of these pathological changes, the original framework of the alveolar wall is preserved in many areas. Intra-alveolar fibrosis may follow as a consequence of damage to the epithelium without severe damage to the underlying basement membrane, which occurs at the stage of organization. Morphological variants of intra-alveolar fibrosis seem to occur not only to the size of the defect of the basement membrane but also to the difference in the stages of evolution at the time the lesion is studied. The epithelium regenerates along the basement membrane in the early stage of re-epithelialization, but grows over the luminal aspect of intra-alveolar fibrous tissue which has been laid on the remaining basement membrane in the late stage. It is speculated that the regeneration of epithelial cells may develop without any association with the basement membrane when a fibrous tissue covers the original basement membrane.

Lung toxicity of paraquat.

Paraquat (1, 1'-dimethyl-4, 4'-bipyridylium), a widely used and effective herbicide, can cause a severe lung injury in both humans and experimental animals. The injury is characterized by initial damage to the alveolar epithelium and capillary endothelium with the development of hemorrhagic pulmonary edema, followed by development of intra-alveolar and interstitial fibrosis. Although the lung is thought to be a primary target organ for paraquat toxicity, the mechanism of action of this herbicide remains to be uncertain. A possible mechanism of selective toxicity to the lung is the accumulation of paraquat in the lung. Data from experimental animals in vivo and in vitro indicate the accumulation of paraquat in the lung by an energy-dependent transport system. However, paraquat does not appear to be selectively concentrated by the lung in human paraquat poisoning. A widely accepted theory regards lipid peroxidation and NADPH depletion as the basic mechanism of paraquat toxicity. According to this theory, paraquat can be reduced by microsomal enzymes and undergoes a cyclic reduction and reoxidation, giving rise to a reactive oxygen radical. This continuous generation of oxygen radicals within the cells of the lung then in turn destroy this tissue through various mechanisms of oxidative damage such as lipid peroxidation and NADPH depletion. However, there are data in the literatures that contradict the active oxygen species theory of paraquat toxicity. Most of these findings show a lack of extensive oxidative damage to the lung. Extent of lipid peroxidation and NADPH depletion is not always correlated with that of lung tissue damage. Moreover, the mechanism of paraquat toxicity leading to the development of lung fibrosis have not been fully elucidated. Recent studies suggest that inflammatory cells such as polymorphonuclear leukocytes and alveolar macrophage, and chemical mediators released by these cells may play an important role in lung fibrosis caused by paraquat. To sum up, the mechanism of selective lung toxicity in paraquat poisoning is not clearly explained by the theory mentioned above. Study on the secondary pulmonary response following cell injury directly caused by paraquat may become a clue to elucidate this mechanism.