Principles of resident tissue macrophages revealed by the eye.

Macrophages are no longer viewed as transient immune passersby but as integral tissue components that regulate physiological homeostasis. Many resident macrophage populations are seeded prenatally and maintained locally, whereas others are progressively replaced by monocytes, yet the rules governing this divergence remain unclear. The eye provides a natural experiment because anatomically distinct compartments harbor macrophages that perform essential functions but differ sharply in persistence under shared systemic conditions. We synthesize evidence that ocular macrophages regulate matrix homeostasis, lipid metabolism, neuroimmune signaling, vascular support, and barrier control. Despite their physiological importance, macrophage populations with divergent persistence concentrate within millimeters, revealing that local tissue context constrains their self-maintenance durability. We introduce establishment kinetics-an organizing framework defined by when and whether populations acquire durable self-maintenance. Four kinetic classes emerge: permanent (lifelong persistence), juvenile-established (stabilized post-early replacement), adult-established (stabilized post-prolonged replacement), and time-limited (never stabilizing). Across ocular and non-ocular tissues, three principles follow: establishment timing does not predict equilibrium density, resident identity does not ensure durability, and kinetic class is independent of anatomical proximity or immune privilege. This framework shifts focus from origin and replacement to establishment kinetics, providing a predictive structure for understanding resident tissue macrophage behavior across homeostasis, aging, and disease.