When a cell releases a signal that acts on neighboring cells in the same tissue — local communication, not systemic circulation.Distinct from endocrine signaling (travels via bloodstream to act on distant cells) and autocrine signaling (acts on the same cell). IGF-1 produced locally in scalp dermis acts on adjacent follicle keratinocytes and dermal papilla cells parabolically — this local production, not circulating IGF-1 levels, is the therapeutically relevant signal for the follicle. This is why intramuscular IGF-1 injection to raise systemic levels is a different pharmacological intervention from local scalp IGF-1 delivery.