1d). The epithelium of the stomach contains mucus producing mucus neck cells, pepsinogen-producing

gastric chief cells, gastric acid and intrinsic factor producing parietal cells and a variety of hormones (gastrin, serotonin, somatostatin, etc.) producing enteroendocrine cells (Fig. 1e). In the small intestine, cells belonging to the immune system (M-cells), enteroendocrine cells and goblet cells are embedded in a layer of enterocytes (Fig. 1f). M-cells are preferentially located in the epithelium overlying the Peyer’s Patches, which is also called Follicle Associated PI3K inhibitor Epithelium (FAE), and deliver foreign substances to the underlying tissues (mucosa lymphoid) to induce immune responses (Gerbert et al., PD0332991 1996). M-cells, however, are also a potential portal for nanoparticles. The epithelium of the large intestine consists of enterocytes and goblet cells. When different cell types adjoin the barrier function of the epithelium is altered because the location and structure of these junctions differ

between the cell types (Eom and Choi, 2009). All epithelia reside on a basal membrane, which separates them from the underlying connective tissue containing capillaries, lymph vessels, lymph follicles and peripheral nerves. To reach the systemic circulation by capillaries NMs have also to cross the basal membrane and the connective tissue. Epithelia can be permeated either by passage through the cells (transcellular) or by passage between the cells (paracellular).

Physiological methods to evaluate interactions with biological barriers and to predict the effect of nanoparticles are highly demanded. Studies addressing permeation usually use transwell systems, where cells are cultured on filters. Moreover, diffusion-cells can be used to evaluate the penetration/permeation of NMs across excised tissues (Sudhakar et al., 2006). Studies on cell monolayers show that polystyrene particles can readily permeate the alveolar epithelium (Yacobi et al., 2008). By contrast, the rate of permeation of enterocyte (Caco-2 cell) monolayers by polystyrene particles without surface coating appears low (Geiser et al., 2005 and Pietzonka et al., 2002). Gaumet et al. (2009) showed that small polystyrene particles were observed intracellularly Aldol condensation in Caco-2 cells. Also TiO2 nanoparticles appear to cross Caco-2 monolayers without disruption of junctional complexes and without causing cytotoxicity (Koeneman et al., 2010). Since the plasma membrane of the cells forming the epithelial barrier is lipophilic, lipophilic substances are taken up passively by the transcellular route whereas hydrophilic drug compounds use the paracellular route. The penetration area of the paracellular route is extremely small compared to the transcellular route and restricted to polar substances below 1000 D. Paracellular transport is only passive. Nanoparticles are not expected to be able to use the paracellular route, because they are considerably larger than 1000 D.