Practice question

Paracrine signaling involves localized secretion of ligands acting over few cell diameters, contrasting endocrine long-range circulation and autocrine self signaling. Ligands like FGF, Wnt, Hedgehog, TGF-beta bind receptors on nearby cells triggering intracellular cascades guiding differentiation, proliferation, migration. This mode enables morphogen gradients patterning fields, such as limb progress zone or neural tube where concentration interprets fate via thresholds. Diffusion limited by extracellular matrix heparan sulfates, proteolysis, receptor-mediated endocytosis, antagonists ensures short-range effect allowing precise spatial control during organogenesis without systemic hormonal influence or direct cell contact needed for juxtacrine.