Patterns of pulmonary structural remodeling after experimental paraquat toxicity. The morphogenesis of intraalveolar fibrosis.

For a study of the evolution of interstitial and intraalveolar fibrosis, ultrastructural and immunohistochemical observations were made of the lungs of 16 cynomolgous monkeys given 1 or 2 injections of 10 mg/kg of paraquat and sacrificed 2 days to 8 weeks later. At 2-3 days, alveolar epithelial cells were denuded in many areas, and fibronectin was conspicuous in alveolar spaces. At 1 week, fibroblasts and inflammatory cells were migrating through gaps in the denuded epithelial basement membranes; Type II cells were regenerating in some areas. At 3-4 weeks, alveoli developing intraalveolar fibrosis contained many myofibroblasts, collagen fibrils, and small elastic fibers; fibrotic alveolar walls were lined by metaplastic squamous cells and bronchiolar epithelial cells. Spiraling collagen fibrils were found in interstitium but not in alveolar spaces, which suggests that they were formed from breakdown of collagen. Newly formed intraalveolar collagen was mainly Type I. At 8 weeks, intraalveolar fibrosis had led to extensive remodeling, with new glandlike alveoli lined by Type II cells; alveoli without intraalveolar fibrosis had more normal architecture. Thus, intraalveolar fibrosis in paraquattreated lung is mediated by intraalveolar migration of interstitial cells, through gaps in the epithelial basement membranes, after epithelial injury. This is followed by connective tissue synthesis on the luminal side of the epithelial basement membrane, by differentiation of interstitial cells into myofibroblasts and smooth-muscle cells, by incorporation of areas of intraalveolar fibrosis into the interstitium, and by coalescence of alveolar walls. Intraalveolar fibrosis is more important than interstitial fibrosis in the structural remodeling that occurs in paraquattreated lung, because it results in obliteration of alveoli, coalescence of alveolar walls, and loss of functional alveolar-capillary units.     

Intraluminal fibrosis induced unilaterally by lobar instillation of CdCl2 into the rat lung.

Lung injury induced by intratracheal instillation of cadmium chloride (CdCl2) into the rat lung may serve as a model of human interstitial lung disease. In this study, CdCl2 solutions were instilled through a lobar bronchus into the left lung of the rat. Two doses (400 micrograms or 50 micrograms of CdCl2, each in 400 microliters of neutral saline) were used and the morphologic changes occurring during the first 7 days after a single exposure were documented by light and electron microscopy. With the higher dose, inflammatory cells appeared in the alveolar interstitium 1 day after CdCl2 administration. Edema and thickening of the alveolar walls were evident, as were damaged type I epithelial cells and denuded basement membranes. Fibrin was found in the air spaces. Within 2 days, inflammatory cells were seen in large numbers and fibroblasts were observed passing through gaps in the alveolar basement membranes into the air spaces. By 4 and 7 days after CdCl2, various forms of intraluminal fibrosis, including intrabronchiolar budding, mural incorporation, and obliterative changes, were observed. The contralateral lungs had normal-appearing architecture for all the time points investigated. In the lower dose exposure, gradients of alveolar damage were observed in which normal lung, interstitial fibrosis, and/or intraluminal fibrosis were seen within treated lungs. In the mildly damaged regions, interstitial fibrosis predominated, while in the more severely damaged regions, mural incorporation of the convoluted basement membranes was observed. The pulmonary fibrosis that developed appeared to be similar to some human interstitial lung diseases and may offer a system in which to study the regulation of collagen deposition and fibrosis development in these pathologic conditions.     

Increased expression of epimorphin in bleomycin-induced pulmonary fibrosis in mice

Epimorphin was originally identified as a mesenchymal, cell surface-associated protein that modulates epithelial morphogenesis in embryonic organs, whereas pulmonary fibrosis is a process of wound healing, which in part mimics the process of fetal lung development. We investigated the temporal and spatial changes in the distribution of epimorphin protein and expression of its messenger RNA (mRNA) in bleomycin-induced pulmonary fibrosis in mice. Immunohistochemical analysis showed that low levels of epimorphin were present in the bronchiolar, alveolar, and vascular walls of normal adult lungs. However, from Day 7 until Day 28 after bleomycin treatment, increasing levels of epimorphin immunoreactivity were detected in the mesenchymal cells and in the extracellular matrix within intra-alveolar fibrotic lesions. Moreover, Northern blots showed corresponding increases in epimorphin mRNA expression. Re-epithelialization of epimorphin-rich intra-alveolar fibrosis was complete by Day 28 after bleomycin, and by Day 56, epimorphin immunoreactivity had declined. In situ hybridization and confocal microscopic studies confirmed expression of epimorphin mRNA by mesenchymal cells situated within early fibrotic lesions, whereas immunoelectron microscopy localized the epimorphin to the endoplasmic reticulum of the mesenchymal cells and to the basement membrane and collagen fibrils in the area. These results suggest that epimorphin may contribute to the remodeling of pulmonary fibrosis via epithelial-mesenchymal interactions.     

Intraluminal fibrosis in interstitial lung disorders.

The histopathologic and ultrastructural features of intraluminal organizing and fibrotic changes were studied in open lung biopsies and autopsy specimens from 373 patients with interstitial lung disorders, including hypersensitivity pneumonitis (n = 44), idiopathic pulmonary fibrosis (n = 92), collagen-vascular diseases (n = 20), chronic eosinophilic pneumonia (n = 10), pulmonary histiocytosis X (n-90), pulmonary sarcoidosis (n = 62), pneumoconioses (n = 25), Legionnaire's disease (n = 5), drug- and toxin-induced pneumonitis (n = 4), radiation-induced pneumonitis (n = 2), lymphangioleiomyomatosis (n = 11), and chronic organizing pneumonia of unknown cause (n = 8). Three patterns of intraluminal organization and fibrosis were recognized: 1) intraluminal buds, which partially filled the alveoli, alveolar ducts and/or distal bronchioles; 2) obliterative changes, in which loose connective tissue masses obliterated the lumens of alveoli, alveolar ducts or distal bronchioles, and 3) mural incorporation of previously intraluminal connective tissue masses, which fused with alveolar, alveolar ductal, or bronchiolar structures and frequently became reepithelialized. All three patterns had common morphologic features, suggesting that, regardless of their severity, they resulted from a common pathogenetic mechanism, ie, the migration of activated connective tissue cells, through defects in the epithelial lining and its basement membrane, from the interstitial into the intraluminal compartment. Intraluminal buds were observed most frequently in hypersensitivity pneumonitis, chronic eosinophilic pneumonia, and organizing pneumonia of unknown cause. Mural incorporation and, to a lesser extent, obliterative changes were observed in most interstitial disorders and were very prominent in idiopathic pulmonary fibrosis. Mural incorporation and obliterative changes play an important role in pulmonary remodeling, especially when several adjacent alveoli and/or other air spaces are involved. Under these circumstances, intraluminal organization can mediate the fusion of adjacent alveolar structures by intraluminal connective tissue.     

Changes in extracellular matrix materials in the uterine myometrium of rats during pregnancy and postparturition

Morphological changes in extracellular matrix materials in the uterine myometrium of rats during pregnancy and postparturition were studied by light and electron microscopy together with immunofluorescence microscopy for type III and IV collagens, fibronectin and laminin. The main components present in late pregnancy were 1) various-sized collagen fibrils, 2) thick elastic fibers adjacent to smooth muscle cells, and 3) continuous and thick basement membranes of hypertrophic smooth muscle cells. These findings are considered to indicate degradation of collagen fibrils and development of elastic fibers and basement membranes of smooth muscle cells. This change in extracellular matrix materials in the late stage of pregnancy may be important in the process of uterine enlargement associated with elasticity and preparation for labor. In the postpartum stage, myofibroblastic interstitial cells were seen to phagocytize collagen fibrils, and elastic fibers accumulated mainly around the bundles of smooth muscle cells. These changes in the postpartum stage are thought to be important for the process in which the uterus returns to the nonpregnant condition. It is suggested that smooth muscle cells participate in regulating the development of their basement membranes and elastic fibers, and that myofibroblastic interstitial cells function by clearing degraded collagen fibrils from the uterine myometrium.     

Changes in Extracellular Matrix Materials in the Uterine Myometrium of Rats during Pregnancy and Postparturition

Morphological changes in extracellular matrix materials in the uterine myometrium of rats during pregnancy and postparturition were studied by light and electron microscopy together with immunofluorescence microscopy for type III and IV collagens, fibronectin and laminin. The main components present in late pregnancy were 1) various-sized collagen fibrils, 2) thick elastic fibers adjacent to smooth muscle cells, and 3) continuous and thick basement membranes of hypertrophic smooth muscle cells. These findings are considered to indicate degradation of collagen fibrils and development of elastic fibers and basement membranes of smooth muscle cells. This change in extracellular matrix materials in the late stage of pregnancy may be important in the process of uterine enlargement associated with elasticity and preparation for labor. In the postpartum stage, myofibroblastic interstitial cells were seen to phagocytize collagen fibrils, and elastic fibers accumulated mainly around the bundles of smooth muscle cells. These changes in the postpartum stage are thought to be important for the process in which the uterus returns to the nonpregnant condition. It is suggested that smooth muscle cells participate in regulating the development of their basement membranes and elastic fibers, and that myofibroblastic interstitial cells function by clearing degraded collagen fibrils from the uterine myometrium. Acta Pathol Jpn 41: 122-132, 1991.     

Anchoring fibrils. A new connective tissue structure in fibrotic lung disease

Electron microscopic studies of lung were made and compared in 17 patients with lung disease (10 with idiopathic pulmonary fibrosis, 3 with collagen--vascular diseases, 3 with sarcoidosis, and 1 with chronic eosinophilic pneumonia) and in 5 control patients. In control patients, the alveolar epithelial cells were normal, and no hemidesmosomes were present between the plasma membranes and the basal laminae. In comparison, cuboidal alveolar epithelial cells were present in 15 of the patients with fibrotic lung disease; in 9 of these the alveolar epithelial cells were multilayered. In 7 of the latter 9 patients (5 with idiopathic pulmonary fibrosis and 2 with collagen-vascular diseases), the basal laminae of the alveolar epithelial cells were attached to the plasma membranes by hemidesmosomes and to the underlying interstitial connective tissue by "anchoring fibrils." These fibrils measured from 4000 to 6000 A in length and from 200 to 600 A in width. One or both ends of the anchoring fibrils inserted into thebasal lamina, often forming arcs through which collagen fibrils and connective tissue microfibrils penetrated. Anchoring fibrils showed a complex pattern of transverse banding, which differed from that of collagen and appeared to be symmetric about the center of the fibril. These anchoring fibrils, which resemble those in normal skin and other tissues, were not found in lungs of control patients. In addition, there was a significant correlation between the severity of the pulmonary fibrosis and the presence of anchoring fibrils. These observations suggest that in severe fibrotic lung disease, anchoring fibrils reinforce the attachment of the basal lamina of multilayered alveolar epithelial cells to interstitial connective tissue.     

An immunohistochemical study of the fibrosing process in paraquat lung injury

Summary 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.     

Immunohistochemical study of metalloproteinases and their tissue inhibitors in the lungs of patients with diffuse alveolar damage and idiopathic pulmonary fibrosis.

Immunohistochemical and confocal microscopic studies of the localization of matrix metalloproteinases (MMPs), their tissue inhibitors (TIMPs), and type IV collagen were made in lung tissues from patients with normal pulmonary histology (n = 3), diffuse alveolar damage (n = 14), and idiopathic pulmonary fibrosis (n = 12). Pretreatment with pepsin revealed otherwise undetectable MMP- and TIMP-immunoreactive sites. In normal lung, MMP-2, MMP-9, TIMP-1, and TIMP-2 were localized in ciliated cells, endothelial cells, pneumocytes, macrophages, and smooth muscle cells; fibroblasts showed a strong reaction only for MMP-2. Only TIMP-2 showed co-localization with type IV collagen. Myofibroblasts and epithelial cells expressed increased reactivity for MMPs and TIMPs in both disorders. The reactivities for MMPs and TIMPs were stronger in diffuse alveolar damage. MMP-2 showed focal co-localization in capillary endothelial and disrupted epithelial basement membranes, suggesting activation of collagenolysis. A protective effect against this lysis was suggested by the extensive co-localization of TIMP-2 with type IV collagen and fibrillar collagens. Alveolar buds showed increased reactivity for MMPs and TIMPs in their lining epithelial cells, myofibroblasts, and their basement membranes; however, their matrices were mostly unreactive. These findings emphasize the complexity of the roles of MMPs and TIMPs in collagen turnover in diffuse alvcolar damage and idiopathic pulmonary fibrosis.     

Experimental lesions in rats corresponding to advanced human asbestosis

Rats inhaling chrysotile asbestos developed a progressive interstitial fibrosis similar in most respects to human asbestosis. The earliest lesions were focal deposits of fibrous tissue in the walls of respiratory bronchioles and alveolar ducts. Later alveolar septa between adjacent bronchioles became progressively thickened to produce lesions with similarities to human honeycombing. The thickened septa between alveoli or "micro-honeycomb" spaces were mainly surfaced with cuboidal epithelial cells although some spaces lined by ciliated columnar epithelium were also found. Transmission electron microscopy of these advanced lesions showed that the cuboidal epithelial cells retained most of the characteristics of type 2 pneumocytes but that they frequently exhibited apical cytoplasmic blebs normally associated with the apocrine secretion of Clara cells. Columnar cells exhibited all stages from fully cilitated to cells with only an occasional cilium among the normal cell surface microvilli. Alveolar or micro-honeycomb spaces frequently contained clusters of pulmonary macrophages with their surface processes interdigitated but with no signs of fusion to giant cells. At more than 18 months after the end of dust inhalation these macrophages contained no chrysotile asbestos. The basement membranes beneath the epithelial layers of thickened septa were irregular and often convoluted as well as being much thicker than normal. Microscopic deposits of calcification were frequently found within the basement membrane material. Some thickened septa were relatively acellular, consisting mainly of masses of collagen fibrils but others were cellular and contained many macrophages, fibroblasts, plasma cells and mast cells. In these advanced lesions extremely little chrysotile asbestos was found and this was present in two sites only. Some chrysotile, always as individual fibrils and usually of short length, was present among collagen fibrils in areas of fibrosis and some was present within the thickened basement membranes.     

The development of alveolar septa in fetal sheep lung. An ultrastructural and immunohistochemical study

The morphogenesis of pulmonary alveolar septa in the sheep was studied by light microscopy, transmission electron microscopy and light microscopic immunohistochemistry for the detection of elastin. The primordia of alveolar septa developed in the glandular stage in areas subjacent to the epithelium, and formed alveolar septa by protruding into the glandular lumina. In their earliest stage, the primordia consisted of groups of fibroblasts, which were associated with elastic fibers and unit collagen fibrils and were surrounded by epithelial basement membrane and by more immature fibroblasts. The fibroblasts in the primordia subsequently became myofibroblasts or smooth muscle cells. In the alveolar zone of the glands, elastic fibers were exclusively found in the primordia of alveolar septa in early developing lung. In early developing lung, wavy, thickened epithelial basement membranes were found in the regions of the glands, which eventually underwent considerable expansion of their surface areas, especially in the primordia of alveolar septa and the bifurcations in the alveolar zones. Areas of fusion of the basement membranes of capillary endothelial cells and epithelial cells in the alveolar zone were found after the formation of the primordia of alveolar septa was accomplished. These areas of fusion were not found in the primordia themselves, but in regions between the primordia. Epithelial cell flattening and differentiation occurred after the formation of the primordia of alveolar septa, and flattening was first observed in the areas of the primordia and the bifurcations of the alveolar zones.     

Intra-alveolar fibrosis of idiopathic bronchiolitis obliterans-organizing pneumonia. Cell-matrix patterns.

Idiopathic bronchiolitis obliterans-organizing pneumonia (BOOP) is characterized by air space fibrosis of unknown origin. Clinical resolution under steroid treatment suggests the removal of the fibrotic lesion. Open lung biopsies of four patients with idiopathic BOOP were studied by immunochemistry and electron microscopy. Three distinct cell-matrix patterns of intra-alveolar bud were found to represent the sequential evolution of the fibrotic process: fibrinoid inflammatory cell clusters in which immunoglobulins and procoagulant factors (fibrinogen, factors VII and X) were identified; fibroinflammatory buds in which desmin-containing fibroblasts were observed migrating, proliferating, and secreting matrix proteins; fibrotic buds in which myofibroblasts organized a loose connective matrix predominantly composed of fibronectin and type III collagen. Extending forms of fibrotic buds may join contiguous alveoli. Fibrotic bud remodeling ability is correlated to the nature and organization of the matrix components but the factors permitting intra-alveolar matrix degradation must be characterized.     

Fine structural changes in idiopathic pulmonary haemosiderosis

Lung biopsies from four children and two adults with idiopathic pulmonary haemosiderosis have been examined by transmission electron microscopy. No qualitative differences were identified between the children and the adults but the changes were more severe in the children. In each case the major damage involved the capillary endothelium and its basement membrane. Capillary endothelial swelling was very noticeable and in one case the endothelium was attenuated but gaps between endothelial cells were very difficult to find. Capillary narrowing and platelet aggregation were common. The capillary endothelial basement membrane showed focal thickening, particularly on the thick side of the air/blood barrier, but no electron dense deposits were identified. Degenerative changes in the alveolar epithelium were not so marked as those in the capillary endothelium and the epithelial basement membrane was normal except for haemosiderin deposition. Haemosiderin was also noted on elastin and within intra-alveolar macrophages. Other secondary changes included mild interstitial oedema and fibrosis. These findings indicate that the major site of damage is the alveolar capillary, but provide no evidence of the cause of the disease.     

Changes in the alveolar connective tissue of the ageing lung

Summary The modifications of the extracellular matrix (ExM) components in the alveolar parenchyma of elderly subjects were investigated using a panel of polyclonal antibodies. The elastic fibers showed a notable decrease along the alveolar walls while type III collagen increased when compared with that of non-elderly controls. No variations of these components were detectable in the alveolar ducts or in the respiratory bronchioli. An increase in the thickness of the alveolar basement membranes was detected in some of the subjects when antibodies against type IV collagen and laminin were used, while antibodies to fibronectin and type V collagen did not reveal any modifications compared with the controls. The modifications revealed in the lungs of the elderly can be related to the alterations of the elastic recoil and pulmonary compliance observed in these subjects.     

Intraluminal fibrosis and elastic fiber degradation lead to lung remodeling in pulmonary Langerhans cell granulomatosis (histiocytosis X).

To evaluate the morphogenesis of lung remodeling in pulmonary Langerhans cell granulomatosis (LCG; previously called histiocytosis X or eosinophilic granuloma), lung tissues obtained by open biopsy from 62 patients with pulmonary LCG were studied by light and electron microscopy. Tissues from 20 patients were also studied by immunohistochemical methods for the detection of fibronectin, elastin, and S-100 protein, and samples from six patients were studied using OKT6 monoclonal antibody. In early stages of pulmonary LCG, the epithelial lining cells were detached and Langerhans cells, inflammatory cells, and myofibroblasts migrated into intraluminal spaces through gaps in the epithelial basement membranes in and around the granulomatous lesions. In late stages, intraluminal fibrosis led to obstruction of alveolar spaces and airways and to coalescence of alveolar walls in and around the granulomatous lesions. Adjacent to these lesions, irregularly dilated alveoli were found with degraded and disrupted elastic fibers. Together, these observations suggest that intraluminal fibrosis and elastic fiber degradation are important processes of lung remodeling in pulmonary LCG.     

Hypersensitivity pneumonitis in man. Light- and electron-microscopic studies of 18 lung biopsies.

Light- and electron-microscopic changes produced by hypersensitivity pneumonitis were analyzed in open lung biopsies taken from 18 patients with chronic forms of the disease. The main changes observed were: alveolitis (both luminal and mural), granulomas, intraalveolar buds, and interstitial fibrosis. The cells infiltrating the alveolar walls were mainly lymphocytes. Occasionally these lymphocytes presented irregularities in the contours of the nuclear membranes and resembled Sézary cells. In one patient, a few lymphocytes were found that resembled "hand-mirror" cells. Intraalveolar macrophages often had a foamy appearance. Granulomas, present in two-thirds of the patients, differed in several respects from those in sarcoidosis: they were smaller, more loosely arranged, and poorly limited; they had a higher content of lymphocytes; and they were located more frequently in alveolar tissue than in the vicinity of bronchioles and vessels. Intraalveolar buds, also present in about two thirds of the patients, were composed mainly of fibroblasts, myofibroblasts, and macrophages in a loose connective tissue that was rich in proteoglycan material. Capillaries and epithelial cells were rarely seen in buds. Alveolar buds appear to develop by a process of disruption of the epithelial lining layer, due to alveolitis, followed by intraalveolar exudation and by subsequent intraalveolar migration of connective tissue cells interacting with macrophages. Severe fibrotic and alveolar epithelial changes were observed in four patients; milder changes were frequent in most other patients. It is concluded that hypersensitivity pneumonitis usually has distinctive morphologic features; these may help to distinguish the resultant pulmonary fibrosis from that due to other causes.     

Pathogenesis of "fibrosis" in interstitial pneumonia: an electron microscopic study

Seven cases in which interstitial fibrosis developed in patients who had acute interstitial pneumonia were studied ultrastructurally to elucidate the pathogenesis of the interstitial thickening seen by light microscopy. Interstitial fibrosis is generally thought to result from fibroblast proliferation and collagen deposition, and this mechanism was confirmed. However, two additional mechanisms that also contributed to the interstitial thickening were identified. One, which was not described previously, involves folding of portions of alveolar septa or collapse of entire alveoli and permanent apposition of their walls. This process occurred in areas that had been denuded of alveolar lining epithelium. Granular pneumocytes attempting to re-epithelialize the denuded basal lamina proliferated over the surface of apposed septa, thereby combining the folded or collapsed alveoli and forming a single thickened septum. The second mechanism involves incorporation of intra-alveolar exudates into alveolar septa. It occurred when granular pneumocytes re-epithelialized along the luminal surface of intra-alveolar debris overlying denuded alveolar epithelial basal laminae. The relative importance of each of these mechanisms in the pathogenesis of interstitial fibrosis and their role in the more common chronic interstitial pneumonias are unknown. However, their recognition may inspire new approaches for the prevention and treatment of interstitial fibrosis.     

肌成纤维细胞在肺纤维化中的来源和作用

目前对于肺纤维化（pulmonary fibrosis ，PF）中肌成纤维细胞的来源还不是很清楚，主要有3种假说：肺部原有成纤维细胞转化成为肌成纤维细胞、肺泡上皮细胞穿越基底膜到达纤维化病灶，经上皮-间充质细胞转化为肌成纤维细胞、血液中的纤维细胞到达纤维化病灶转化为肌成纤维细胞等。肌成纤维细胞在PF的病理进程中扮演着重要的角色，其胶原合成能力强可造成细胞外基质的异常沉积、具收缩性使肺顺应性下降、分泌多种炎性介质加重肺泡上皮损伤。     

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 bromo-deoxyuridine (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 untill 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.     

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.