Practice question

Stem cell niches provide essential extrinsic regulation via secreted morphogens including Wnt family promoting self-renewal and proliferation, Hedgehog patterning tissue boundaries, fibroblast growth factors supporting survival and growth, BMP gradients restricting stemness outside niche, and Notch juxtacrine signaling maintaining undifferentiated state. Intracellular apoptotic executioners such as caspases are downstream intracellular effectors of cell death programs, not secreted extracellular ligands that instruct fate choice through receptors. While niche stress can trigger caspase activation to eliminate damaged stem cells for quality control, caspases themselves do not function as instructive niche signals governing identity or lineage specification.