Extracellular space refers to the part of a multicellular organism outside the cells, usually taken to be outside the plasma membranes, and occupied by fluid. The composition of the extracellular space includes metabolites, ions, proteins, and many other substances that might affect cellular function. For example, neurotransmitters "jump" from cell to cell to facilitate the transmission of an electric current in the nervous system. Hormones also act by travelling the extracellular space towards cell receptors. In cell biology, molecular biology and related fields, the wordextracellular means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid. The term is used in contrast to intracellular. According to the Gene Ontology, the extracellular space is a cellular component defined as: "That part of a multicellular organism outside the cells proper, usually taken to be outside the plasma membranes, and occupied by fluid. For multicellular organisms, the extracellular space refers to everything outside a cell, but still within the organism. Gene products from a multi-cellular organism that are secreted from a cell into the interstitial fluid or blood can therefore be annotated to this term". The composition of the extracellular space includes metabolites, ions, various proteins and non-protein substances, and particles such as extracellular vesicles that might affect cellular function. For example, hormones, growth factors, cytokines and chemokines act by travelling the extracellular space towards biochemical receptors on cells. Other proteins that are active outside the cell are various enzymes, including digestive enzymes, extracellular proteinases and antioxidant enzymes. Often, proteins present in the extracellular space are stored outside the cells by attaching to various extracellular matrix components. In addition, extracellular matrix proteolyticproducts are also present in the extracellular space, especially in tissues undergoing remodelling Extracellular#cite note-1|.
