Absorption epithelial biochemical barrier

Cell lines of both airway and alveolar epithelial origin are available (immortal cells, transformed or carcinoma-derived), the most commonly used being the CALU-3 cell line derived from airway epithelium [37], and the A549 cell line derived from a bronchioalveolar carcinoma [38]. However, these cultured cell lines share only limited similarity in morphology, biochemical characteristics, and barrier properties (low epithelial resistance due to the lack of tight junctions) with the epithelia in vivo. Consequently, the results of absorption and metabolism studies based on cultured cell lines should be interpreted with caution. 

The primary cell culture model is a more valid model for the study of absorption and transport processes of a drug via the pulmonary route. It provides a tight epithelial barrier with morphological and functional properties resembling those of the in-vivo condition. Primary alveolar epithelial cells from rats [39], rabbits [40] and humans [41] which display morphological and biochemical characteristics similar to the native epithelium have been isolated and can be used for drug transport studies. 

The epithelium is supported underneath by lamina propria and a layer of smooth muscle called muscularis mucosa (3-10 cells thick). These three layers, i.e., the epithelium, lamina propria, and muscularis mucosa, together constitute the intestinal mucosa.On the apical surface, the epithelium along with lamina propria projects to form villi. The cell membranes of epithelial cells that comprise the villi contain uniform microvilli, which give the cells a fuzzy border, collectively called a brush border. These structures, although greatly increase the absorptive surface area of the small intestine, provide an additional enzymatic barrier since the intestinal digestive enzymes are contained in the brush border. In addition, on the top of the epithelial layer lies another layer, the UWL, as previously described. The metabolic and biochemical components of the epithelial barrier will be discussed. 

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