Practice question

Paracrine factors act locally over microns, not systemically via bloodstream like hormones. They rely on diffusion, facilitated transport, cytonemes or exosomes over 10-100 cell diameters, rapidly degraded or sequestered by proteoglycans and antagonists like Noggin, Chordin. Endocrine hormones such as estrogen, thyroxine travel meters via blood, requiring high stability and low clearance. Paracrine mode involves ligand-receptor binding triggering MAPK, Smad cascades guiding organogenesis, wound healing, stem cell niches. Morphogen gradients emerge precisely because signaling range limited, whereas endocrine provides uniform concentration distinguishing local developmental patterning versus organism-wide homeostasis and metabolic control mechanisms.