Cytosis

Cytosis is a collective term in cell biology for vesicle-mediated transport of materials into and out of the cell. It encompasses endocytosis, exocytosis, and, in some contexts, transcytosis, which moves substances across the cell from one membrane domain to another.

Endocytosis describes uptake of external particles and fluids via vesicle formation at the plasma membrane. Types include phagocytosis (ingestion of large particles), pinocytosis (cell drinking of fluids), and receptor-mediated endocytosis (specific uptake via membrane receptors). Most forms depend on energy, cytoskeletal remodeling, and membrane curvature factors such as clathrin or caveolin, and often involve dynamin to sever vesicles.

Exocytosis releases intracellular vesicle contents to the extracellular space. This process is essential for secretion of hormones and neurotransmitters, membrane protein and lipid turnover, and delivery of enzymes. Exocytosis occurs in constitutive and regulated forms, and relies on SNARE and related proteins to fuse vesicles with the plasma membrane.

Cytosis dynamics enable nutrient uptake, pathogen clearance by phagocytes, antigen presentation, and maintenance of membrane composition. Dysregulation of vesicular transport is associated with diseases, and many pathogens exploit endocytic pathways to enter cells.

The term cytosis is also used as a suffix in hematology to denote an increased number of cells, such as erythrocytosis (high red blood cells), leukocytosis (high white blood cells), and thrombocytosis (high platelets).