Practice question

Paracrine signaling is short-range communication where secreted proteins travel tens to hundreds of micrometers through extracellular matrix, binding receptors on nearby cells. It involves ligand-receptor interaction, gradient formation, endocytic clearance and threshold-dependent gene activation, exemplified by FGF, Hedgehog, Wnt and TGF-beta families patterning embryonic fields. Communicating with distant organs requires endocrine signaling via bloodstream, such as insulin, estrogen or growth hormone traveling systemically and acting at low concentration. Paracrine cannot act systemically due to dilution, matrix binding and rapid degradation by proteases. Therefore systemic distant organ communication is not paracrine characteristic, distinguishing it from endocrine mode.