transepithelial

Transepithelial refers to the movement of substances across an epithelium, the layer of cells that lines cavities and surfaces of the body. This term emphasizes the net flux from one side of the epithelial sheet to the other, typically from the apical (lumen-facing) surface to the basolateral (tissue-facing) surface, or in the reverse direction. Epithelium forms a selective barrier through tight junctions, cell membranes, and transport proteins, enabling controlled absorption, secretion, and barrier function.

Transepithelial transport can occur via transcellular routes, where solutes pass through the epithelial cells using channels, carriers, and pumps located at the apical and basolateral membranes; or via paracellular routes, where solutes pass between cells through tight junctions. Net transepithelial transport is the combination of these pathways. Transport can be passive, driven by concentration or electrochemical gradients, or active, requiring energy either directly (primary active transport) or indirectly (secondary active transport).

Applications and examples: in the intestine, nutrient absorption relies on transepithelial transport (e.g., glucose uptake via SGLT1 on the apical membrane and GLUT2 on the basolateral side). In the kidney, transepithelial reabsorption and secretion regulate solute and water balance. In the respiratory and other epithelia, ion transport maintains fluid and mucus homeostasis. The integrity of transepithelial barriers is commonly assessed in research by measuring transepithelial electrical resistance (TEER) and solute flux in Ussing chamber experiments.

Clinical relevance: impaired transepithelial transport or barrier dysfunction is implicated in diseases such as inflammatory bowel disease, cystic fibrosis, and certain kidney disorders. Understanding transepithelial mechanisms informs drug delivery strategies, intestinal absorption, and the development of therapies targeting epithelial transport proteins.