Parakrin

Parakrin (paracrine) signaling refers to a mode of cell communication in which cells secrete signaling molecules that affect nearby target cells within the same tissue or organ, without entering systemic circulation. The term derives from Greek para- meaning beside and krinein meaning to separate or discern.

Locally released mediators diffuse through the extracellular space and bind to receptors on adjacent cells. Common paracrine mediators include growth factors such as epidermal growth factor (EGF), transforming growth factor-beta (TGF-β), and vascular endothelial growth factor (VEGF); cytokines and chemokines; prostaglandins; and nitric oxide. These signals often act over short distances and short time frames, but can influence multiple nearby cell types.

Paracrine signaling is distinct from autocrine signaling, where a cell responds to signals it itself releases, and endocrine signaling, where signals travel through the bloodstream to distant targets. Paracrine actions typically occur locally, with rapid diffusion and degradation shaping the range and duration of the response. Some signals can have overlapping autocrine and paracrine effects, depending on context.

In biology, paracrine communication coordinates processes such as development, tissue homeostasis, wound healing, and immune responses by regulating cell proliferation, differentiation, and migration. Dysregulation of paracrine pathways is implicated in diseases including cancer and chronic inflammation. Therapeutically, strategies sometimes aim to modulate paracrine signals, for example by inhibiting VEGF or TGF-β signaling to limit disease progression.