lexocytose

Lexocytose, often written exocytosis, is the cellular process by which intracellular vesicles fuse with the plasma membrane to release their cargo into the extracellular space. It is essential for secretion of neurotransmitters, hormones, enzymes, and extracellular matrix components, and it also contributes membrane material to the cell surface.

Two major modes are recognized. Constitutive exocytosis operates continuously and independently of external signals, delivering lipids and proteins to the plasma membrane and maintaining surface composition. Regulated exocytosis is triggered by specific signals, frequently a rise in cytosolic calcium, and mediates rapid, stimulus-secretion responses such as neurotransmitter release at synapses or hormone discharge from endocrine cells.

Mechanistically, vesicles bud from the endomembrane system, typically the Golgi apparatus, and are transported to the plasma membrane along cytoskeletal tracks. Docking is mediated by Rab GTPases and tethering factors, followed by SNARE-mediated fusion of the vesicle and plasma membranes. In regulated exocytosis, a Ca2+ sensor such as synaptotagmin initiates fusion. The process releases vesicle contents and may contribute membrane to the cell surface, which is later balanced by endocytosis.

Beyond neurons and endocrine cells, exocytosis supplies extracellular enzymes, mucins, and membrane proteins in a variety of cell types, and plays a role in immune responses and development.

Defects in exocytosis can underlie secretory diseases, neurodegenerative conditions, and immune dysfunction. Research employs imaging techniques like fluorescence and electron microscopy, along with genetic and pharmacological tools, to dissect trafficking steps and SNARE regulation.