Alveolar epithelium

At the distal respiratory site, the alveolar epithelial cell layer is much flatter (0.1 -0.5 pm) and composed of two major cell types, squamous type I and agranular type II pneumocytes. Type I pneumocytes are non-phagocytic and highly flattened cells with broad and thin extensions. They occupy -95 % of the alveolar luminal surface, although they are less numerous than type II cells. The remaining surface is occupied by type II pneumocytes, which have blunt microvilli and contain multivesicular bodies [3, 11]. 

Type I pneumocytes, joined with endothelial cells by fused basement membranes, offer a very short airways-blood pathway for the diffusion of gases and drug molecules. They are known to contain numerous endocytotic vesicles which play an important role in the absorption process of proteins and transcellular movement of transporters [12, 13]. The functions of type II pneumocytes are well studied and include 

Hydrophilic surfactant proteins A (SP-A) and D (SP-D), secreted by type II pneumocytes, interact specifically with a wide range of microorganisms and play important roles in the innate, natural defense system of the lung [16]. Both mRNA and protein levels of SP-A and SP-D increase dramatically in response to lung infection, injury and endotoxin challenge [17]. Type II pneumocytes also express class II major histocompatibility complex (MHC) antigens and intracellular adhesion molecule (ICAM-1), which may facilitate pulmonary immune responses [15]. 

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From these data one can calculate the effective radius of the pores through which solutes diffuse across the junctional strands (Fig. 16). At day 3, the pore radius was —5.5 A. This correlates with pore radii of —10 A for dog alveolar epithelium (Taylor and Gaar, 1970) and 5 and 8 A for rabbit and bullfrog gallblad-... 

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. 

Rat, Rattus sp. 27 mg/m3 air for 1-2 h Fatal death by asphyxiation lung edema necrosis of alveolar epithelium 7... 

Hohansson, A., P. Camner, and B. Robertson. 1981. Effects of long-term nickel dust exposure on rabbit alveolar epithelium. Environ. Res. 25 391-403. 

No deaths in 72 h marked reduction of acid and alkaline phosphatase activity in alveolar epithelium of lung (Boudreau and Nadeau 1987)... 

Sakagami M, Omidi Y, Campbell L, Kandalaft LE, Moris CJ, Barar J, Gumbleton M (2006) Expression and transport functionality of FcRn within rat alveolar epithelium a study in primary cell culture and in the isolated perfused lung. Pharm Res 23 270-279. 

Yang X, Ma JKA, Malanga CJ, Rojanasakul Y (2000) Characterization of proteolytic activities of pulmonary alveolar epithelium. Int J Pharm 195 93-101. 

Keywords Pneumocytes Pulmonary drug delivery Pulmonary metabolism, Alveolar epithelium... 

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... 

Newman GR, Campbell L, von Ruhland C, Jasani B, Gumbleton M (1999) Caveolin and its cellular and subcellular immunolocalisation in lung alveolar epithelium implications for alveolar epithelial type I cell function. Cell Tissue Res 295(1) 111-120... 

Kang BH, Crapo JD, Wegner CD, Letts LG, Chang LY (1993) Intercellular adhesion molecule-1 expression on the alveolar epithelium and its modification by hyperoxia. Am J Respir Cell Mol Biol 9(4) 350-355... 

Center DM (2000) A role for the alveolar epithelium in recruitment of mononuclear cells into the lung. J Clin Invest 106(6) 741-743... 

Crandall ED, Kim KJ (1982) Transport of water and solutes across bullfrog alveolar epithelium. J Appl Physiol 52(4) 902-909... 

Campbell L, Abulrob AN, Kandalaft LE, Plummer S, Hollins AJ, Gibbs A, Gumbleton M (2003) Constitutive expression of p-glycoprotein in normal lung alveolar epithelium and functionality in primary alveolar epithelial cultures. J Pharmacol Exp Ther 304(1 ) 441 152... 

The intention to study transport processes at pulmonary epithelia, however, raised two particular problems (i) the apical side of these epithelia is typically in contact with air rather than with a liquid and (ii) in order to maximize the surface area, the lungs have a complex treelike structure, ending in millions of tiny alveolar bubbles. The total surface area of the human alveolar epithelium is almost half of that of the intestines (100-120 m2), with its macroscopic appearance resembling a sponge, and it is virtually impossible to use such a tissue for transport experiments in a diffusion-chamber setup. 

Lehr CM (2001) In vitro models of intestinal and alveolar epithelium cultures in pharmaceutical research. Altex 18 59-63... 

Kim KJ, Crandall ED (1983) Heteropore populations of bullfrog alveolar epithelium. J Appl Physiol 54 140-146... 

Ectopeptidases of the alveolar epithelium are also involved in the regulation of pulmonary surfactants [24], The cell-surface enzymes on monocyte-... 

The alveolar epithelium is thinner and leakier than the bronchial epithelium. 

The alveolar epithelium consists of so-called Type I and Type II cells. Type I cells cover over 90% of the alveolar surface, have a large surface, and are thin. Type II cells are larger in numbers but are small. Therefore, they cover only about 7% of the surface of the alveoli. Type II cells produce the phospholipids that make up the surfactant layer. 

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