Transcytosis

Transcytosis is a cellular transport process by which macromolecules are moved across the interior of a cell, from one plasma membrane to the opposite membrane, effectively crossing the cytoplasm without being degraded in lysosomes. It comprises endocytosis at the donor surface, vesicular trafficking through the cytoplasm, and exocytosis at the receiving surface. The process is prominent in polarized cells such as intestinal epithelial and endothelial cells and in placental tissue.

Mechanisms include receptor-mediated routes, often involving clathrin-coated pits or caveolae, and non-clathrin pathways such as macropinocytosis. Cargo is sorted in endosomal compartments, may be carried in recycling endosomes, and is released by fusion with the opposite membrane. Specific receptors guide transcytosis for particular molecules: the neonatal Fc receptor (FcRn) transports IgG across epithelial barriers; the transferrin receptor mediates iron-bound transferrin; albumin and other proteins can also be trafficked via transcytotic carriers.

Physiological roles: transcytosis enables maternal IgG transfer to the fetus in the placenta, absorption of immunoglobulins in mucosal tissues, nutrient or hormone transport, and maintenance of the blood-brain barrier by certain endothelial pathways.

Clinical and biotechnological relevance: pathogens can exploit transcytosis to cross barriers; genetic or functional defects can affect IgG distribution. The process is also explored for delivering drugs and vaccines across barriers, by designing cargo and ligands that engage specific transcytotic routes.