Parakriiniset

Parakriiniset refers to a mode of cell signaling in which cells release regulatory molecules that act on nearby target cells within the local tissue environment. In paracrine signaling, the signaling factors are secreted into the extracellular space and diffuse to neighboring cells that express the appropriate receptors. The range of action is typically short and localized, in contrast to endocrine signals that travel through the bloodstream to distant sites. Paracrine signaling is distinct from autocrine signaling, where the same cell that releases the factor is its own target, and from juxtacrine signaling, which requires direct cell-to-cell contact.

Common paracrine mediators include growth factors such as vascular endothelial growth factor (VEGF), fibroblast growth factors (FGFs), and platelet-derived growth factor (PDGF); cytokines such as interleukins; morphogens like Wnt and bone morphogenetic proteins (BMPs); and soluble gases such as nitric oxide. These molecules regulate a wide range of biological processes, including development, tissue repair, angiogenesis, immune responses, and maintaining tissue homeostasis. During development, paracrine signals help create concentration gradients that instruct neighboring cells to adopt specific fates. In adult tissues, paracrine factors contribute to wound healing and regeneration.

Mechanistically, paracrine signals bind to receptors on nearby cells and trigger intracellular signaling cascades that alter gene expression and cellular behavior. In addition to soluble factors, signaling can occur through extracellular vesicles or matrix-anchored proteins that present signals to adjacent cells. Dysregulation of paracrine signaling has been linked to diseases such as cancer, chronic inflammation, and fibrosis.