Paracellulär

Paracellulär (paracellular) describes transport or movement that occurs between adjacent epithelial or endothelial cells, in contrast to transcellular routes that pass through the cell membranes of individual cells. The paracellular route relies on the paracellular space at the apical junctional complex, mainly tight junctions, or to a lesser extent, adherens junctions, to control what can pass from one side of a cellular layer to the other.

Tight junctions consist of transmembrane proteins like claudins, occludin, and tricellulin and cytoplasmic scaffolding proteins such as ZO-1, ZO-2, and ZO-3. The composition and phosphorylation state of these components determine permeability, affecting size- and charge-selective passage. The paracellular pathway typically favors small ions and water, with transport provided by solvent drag or osmosis.

Permeability is modulated by physiological regulators (neural, hormonal, inflammatory signals) and pathophysiological conditions. Cytokines such as TNF-α and interferons can increase paracellular flux by disrupting tight junctions; dietary factors and certain drugs can also alter it. Measurement is commonly done by transepithelial electrical resistance (TEER) and by flux assays using marker molecules (e.g., mannitol, inulin).

Physiologically, paracellular transport is important in organs with barrier or absorptive functions, such as the intestinal epithelium, renal tubules, and the blood–tissue barriers where tight junctions regulate solute and water movement. Clinically, increased paracellular permeability is associated with inflammatory diseases and metabolic disorders, while reduced permeability can impair nutrient absorption.

Paracellulär is the German term for paracellular and is used in bilingual discussions of epithelial physiology. It contrasts with transcellular transport, which involves endocytosis, diffusion across the cytoplasm, and exocytosis.