Practice question

Juxtacrine signaling requires direct membrane apposition where ligand retained on surface of one cell activates receptor on neighboring cell without secretion. Canonical example is Notch-Delta: Delta family ligand on signaling cell binds Notch receptor, inducing proteolytic cleavages by ADAM and gamma-secretase releasing intracellular domain that activates transcription via CSL. BMP-Wnt interaction represents paracrine crosstalk where secreted morphogens antagonize or synergize via extracellular binding, diffusing at distance, not contact-dependent signal. TGF-beta-Smad and FGF-Erk also represent diffusible paracrine with intracellular cascades. Therefore juxtacrine hallmark is cell-contact dependence without extracellular gradient, typified by Notch-Delta lateral inhibition mechanism controlling fate.