Egzitosis

Egzitosis, or exocytosis, is the cellular process by which a cell exports materials to the extracellular space through the fusion of vesicle membranes with the plasma membrane. It includes constitutive exocytosis, a continuous delivery of membrane and secretory proteins, and regulated exocytosis, which is triggered by specific stimuli such as an increase in intracellular calcium.

In the mechanism of egzitosis, secretory vesicles bud from intracellular compartments such as the trans-Golgi network or endosomes, then move to the plasma membrane. Vesicles are tethered and docked through Rab GTPases and other tethering factors. Fusion is driven by SNARE proteins: vesicle-associated v-SNAREs (for example, synaptobrevin/VAMP) and target-membrane t-SNAREs (such as syntaxin and SNAP-25). A rise in intracellular Ca2+ activates calcium sensors like synaptotagmin, promoting SNARE complex assembly and membrane fusion. The merged membranes form a fusion pore through which the vesicle's contents are released. After fusion, the vesicle membrane becomes part of the plasma membrane and excess membrane is retrieved by endocytosis to maintain surface area.

Egzitosis serves essential roles across cell types. In neurons, it mediates neurotransmitter release; in endocrine cells, it drives hormone and enzyme secretion. Lysosomal and other secretory pathways contribute to plasma membrane repair and remodeling, and immune cells use exocytosis to release cytotoxic mediators. Defects in egzitosis can contribute to neurological disorders, metabolic diseases such as diabetes, and immune system dysfunctions.