Extracellular

Extracellular refers to anything located outside the cell membrane or outside the cytoplasm, including the extracellular space and extracellular matrices that surround cells in tissues. In multicellular organisms, most physiological processes occur in the extracellular compartment, where cells exchange nutrients, signals, and waste with their surroundings.

The extracellular matrix is a complex network of macromolecules that provides structural support and biochemical cues to cells. It comprises fibrous proteins such as collagen and elastin, glycoproteins such as fibronectin and laminin, and proteoglycans that form hydrated gels. The ECM influences cell adhesion, migration, growth, and differentiation and is organized into various basement membranes and pericellular matrices that compartmentalize tissues.

Extracellular fluids include interstitial fluid, blood plasma, lymph, and cerebrospinal fluid. These fluids contain water, ions, nutrients, metabolites, signaling molecules, and waste products. Transport between intracellular and extracellular spaces occurs primarily by diffusion, facilitated transport, and bulk flow, enabling nutrient delivery and waste elimination while maintaining osmotic and pH balance.

Extracellular signaling involves molecules released into the extracellular space that act on nearby or distant cells. Hormones, neurotransmitters, cytokines, and growth factors bind to receptors to regulate gene expression, metabolism, and cell behavior. The ECM also serves as a reservoir for signaling molecules and modulates signaling through remodeling and mechanical cues.

Clinical relevance includes edema from fluid imbalance, fibrosis from excess ECM deposition, and tumor progression influenced by ECM remodeling and the tumor microenvironment. Understanding extracellular biology is essential for fields ranging from physiology to pathology and tissue engineering.