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The transport mechanisms of materials through epithelium. Paracellular transport (a), transcellular transport by diffusion (b), facilitated diffusion (c), transcytosis (d), endocytosis (e), active transport (f), and finally, efflux (g). Paracellular transport, is related to tight junction integrity, while transport mechanisms (b) to (f) are all loosely described as transcellular transport mechanisms. Efflux (g) will not be discussed in this review.
