Type I alveolar cells

More specifically, the blood-gas interface consists of the alveolar epithelium, capillary endothelium, and interstitium. The alveolar wall is made up of a single layer of flattened type I alveolar cells. The capillaries surrounding the alveoli also consist of a single layer of cells — endothelial cells. In between the alveolar epithelium and capillary endothelium is a very small amount of interstitium. Taken together, only 0.5 pm separates the air in the alveoli from the blood in the capillaries. The extreme thinness of the blood-gas interface further facilitates gas exchange by way of diffusion. 

Stephens et al have shown that, in rats, the degenerative changes in Type I alveolar cells occur after exposure to ozone at concentrations as low as 0.2 ppm for 3 h and that cells are replaced by Type II alveolar cells beginning a day after the exposure. With electron microscopy, Bils noted that the swelling of the epithelial alveolar lining cells of mice occurred after exposure to ozone at 0.6-1.3 ppm for 4 h. These changes... 

Despite the fact that most of the alveolar surface is composited of alveolar epithelium, three primary types of cells are present in the alveoli type I alveolar cells, type II alveolar cells, and alveolar macrophages. Type I alveolar cells are also referred to as squamous pulmonary epithelial cells and are the continuous lining of the alveolar sac. Type II alveolar cells are also referred to as septal cells. Type II alveolar cells secrete the alveolar fluid that is necessary to keep the surface moist and to maintain surface tension of the alveolar fluid surface tension is necessary to keep the alveoli from collapsing. Alveolar fluid is a suitable environment for proteins when compared to the low pH and high protease levels associated with the intestine... 

In the A region the tight junction gap between type-I alveolar cells is reported as 1 nm. Other pores with equivalent radii of about 10 nm have also been identified. Consequently the permeability of the paracellular route is much greater than seen with other membranes. Large molecules up to 150 kDa are reported to be absorbed to a small extent into the bloodstream after pulmonary administration. 

Once a drug aerosol has made its way through the conducting airways to deposit in the deep lung, the major barriers to entering the body are the 0.15 pm layer of type I alveolar cells that are covered by a very thin layer of epithelial lining fluid consisting mainly of surfactant and the relatively permeable endothelium of the alveolar capillaries. Alveolar cells have so called... 

The type I alveolar cell constitutes over 90% of the alveolar surface. This cell is highly differentiated and very flat. It provides the thin surface through which gas exchange with the pulmonary capillaries occurs. It is separated from the endothelial cells that line the capillary lumen by a common basement membrane. 

After subcutaneous injection of 150 mg pilocarpine per kg rat, type I alveolar cells showed no detectable changes at any stage of the experiment (Gold-enberg et al. 1969). 

Type II pneumocytes can be identified at the hght microscope level by antibodies against cyto-keratin 18 (Schlichenmaier et al. 2000). This intermediate filament was thereby predominantly lo-cahsed in the apical compartment and the intensity of the immunostaining depends on the stage of development. Type I alveolar cells as well as alveolar... 

Type II alveolar epithelial cells synthesize surfactant, which reduces surface tension, stabilizes the alveoli, atxl allows the inflalion of small alveoli at low air pressures. In addition, type II alveolar celk act as stem celk (precursors) for type I alveolar cells, which ate the primary cdlular components of the alveolar epithelial lining. 

The human alveolar epithelium consists of two cell types type I (alveolar epithelial type I [ATI], pneumocyte I) and type n (alveolar epithelial type II... 

Figure 11.3 Permeability to FITC-labelled dextrans across rat (O, [94]) and human ( , [66]) alveolar type I (ATI) cell-like...

Dobbs LG, Gonzalez R, Matthay MA, Carter EP, Allen L, Verkman AS (1998) Highly water-permeable type I alveolar epithelial cells confer high water permeability between the airspace and vasculature in rat lung. Proc Natl Acad Sci USA 95(6) 2991-2996... 

N02 is a relatively insoluble deep lung irritant capable of producing pulmonary edema. The type I cells of the alveoli appear to be the cells chiefly affected on acute exposure. At higher exposure, both type I and type II alveolar cells are damaged. Exposure to 25 ppm of N02 is irritating to some individuals 50 ppm is moderately irritating to the eyes and nose. Exposure... 

The rate of protein clearance has been estimated as 10% of the rate of fluid clearance from alveoli [173]. IgG clearance is probably mediated by FcRn transcytosis in distal type I alveolar epithelium and more proximal bronchial epithelium. Type I alveolar epithelium and bronchial epithelium contain the necessary subcellular structures for FcRn-mediated transcytosis vesicles, membrane invaginations, caveolae, and clathrin-coated pits [173,174], FcRn mRNA is expressed in lung although the cell types and locations have not yet been determined [112], Moreover, primary alveolar epithelial monolayer cell cultures express functional FcRn [173], plgA-R/SC transcytosis is thought to contribute little to distal (alveolar) airway IgG transport but might mediate more proximal (bronchial or bronchiolar) IgA transport [173], Uptake of an aerosolized IgG Fc-erythropoietin fusion molecule and subsequent erythropoietin-induced reticulocytosis has been demonstrated in human and nonhuman primates [175],... 

Gonzalez RF, Dobbs LG (1998) Purification and analysis of RTI40, a type I alveolar epithelial cell apical membrane protein. Biochim Biophys Acta 1429(1) 208-216... 

Although interspecies differences can be circumvented through the use of primary cultures of human alveolar epithelial cells, routine use of these cells is limited by availability.58,60 Primary culture of rat type II alveolar epithelial cells is economical and demonstrates high reproducibility, and its phospholipid secretion is similar to that of human type II cells.43 P-gp is expressed in both human and rat type I cells but not in freshly isolated rat type II cells.61 It is assumed that alveolar type II cells are progenitors for regenerating type I cells in vivo, and type II cells in culture lose their cuboidal appearance, lamellar bodies, and microvilli, and the number of surfactant proteins decrease. Monolayers are formed in 5 to 8 days, and the transdifferentiation to type I-like cells is complete within 7-8 days, characterized by the development of attenuations, tighter junctions, and increasing expression of... 

The pathogenesis of pulmonary fibrosis is presumably related to initial loss of alveolar type I epithelial cells and endothelial cells. However, the dysregulated repair of pulmonary fibrosis is followed by persistence of inflammation. This is followed by proliferation of type II cells, recruitment and proliferation of endothelial cells and fibroblasts, and deposition of extracellular matrix leading to end-stage alveolar and interstitial fibrosis. These events involve the complex and dynamic interplay between diverse immune effector cells and cellular constituents of the alveolar-capillary membrane and interstitium of the lung. Interaction of these diverse cell populations and the cytokines that they produce culminate in chronic inflammation, angiogenesis, fibroproliferation, and deposition of extracellular matrix. 

Alveolar epithelium consists of three types of cells. Roughly 98 % of the alveolar surface is covered by squamous (type I) epithelial cells, from which thin sheets of cytoplasm extend to cover large areas of surface of even several alveoli (Haies et al. 1981). Type II epithelial cells, though more numerous but compact, occupy only 2 % of the alveolar surface. Type III cells, also called alveolar brush cells, are rare. 

Fig. 95. Perikaryon of a type I alveolar epithelial cell from an unmedicated female rat (breeder Winkelmann, Borchen-Kirchborchen). Fixed under methitural anaesthesia by intratracheal instillation of 2.5 % glutaraldehyde in phosphate buffer (pH 7.4) before opening the thorax. Postfixation with 1 % osmium tetroxide m phosphate buffer (pH 7.4). Contrasted en bloc for 12 h with 0.5 % uranyl acetate in 70 % ethanol. Embedded in a 2 8 mixture of methyl and butyl methacrylate. Sectioned at 50 nm. Lead citrate after Reynolds (1963). Plate 13/07...

The lungs of two individuals dying acutely of NH3 inhalation showed marked swelling and inhibi-tional oedema of the type I alveolar epithelial cells (Burns et al. 1983). 

Plutonium particles not phagocytized by alveolar macrophages and removed from the lung are found in type I alveolar epithelial cells, which phagozytize plutonixun particles within a few hours after deposition (Sanders and Adee 1970). These cells appear relatively radioresistant. Fine structural studies of lung exposed to radiation doses up to 13.000 rads from Pu showed no evidence of type I cell detachment or removal (Sanders et al. 1971). 

Zinc salts isolated from the respirable range particulate material EHC-93 instilled intratracheally into Swiss mice induced rapid focal necrosis of type I alveolar epitheUal cells followed by inflammation and elevation of protein levels in lavage fluid over a 2-week period (Adamson et al. 2000). 

Type II alveolar cells proliferate to replace the Type I epithelium, leading to less effective pulmonary air exchange. 

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