Transcytosen

Transcytosen, also known in English as transcytosis, is a cellular transport process that moves macromolecules across polarized cells such as endothelial or epithelial cells. The cargo is internalized at one membrane domain by endocytosis, shuttled through vesicular compartments across the cytoplasm, and released at the opposite surface by exocytosis. Transcytosis enables signals, nutrients, or antibodies to cross tissue barriers without exposing intracellular components to the extracellular milieu.

Several routes mediate transcytosis. Receptor-mediated transcytosis uses specific surface receptors to bind ligands, cluster in endosomes, and deliver cargo to the opposite membrane; examples include the transferrin receptor, the polymeric immunoglobulin receptor (pIgR) for secretory IgA and IgM, and the neonatal Fc receptor (FcRn) for IgG. Adsorptive transcytosis relies on charge interactions with the cell surface, while caveolae-dependent pathways involve caveolin proteins. In many tissues, routing through endosomes must avoid lysosomal degradation.

Transcytosen plays a central role in physiology and medicine. It underpins maternal–fetal IgG transfer via FcRn, mucosal immunity through secretory IgA transport by pIgR, and the movement of albumin and other plasma proteins across endothelial barriers. It also affects drug delivery, as researchers design antibodies or nanoparticles to exploit transcytotic routes to reach target tissues. Regulation of transcytosis is complex and can be limiting; efficiency depends on receptor expression, intracellular sorting, and barrier integrity.