Epicardium is required for cardiac seeding by yolk sac macrophages, precursors of resident macrophages of the adult heart.

A subset of macrophages that reside in adult tissues originate from the fetal yolk sac, while others derive from circulating monocytes. These ontologically different macrophage subsets have distinct roles in tissue injury responses, with the embryonic population overall having beneficial activity in cardiac repair. Here we show that fetal yolk macrophages are recruited to a niche within and just below the epicardium, the mesothelial covering of the heart. The epicardium was required for establishment of yolk sac macrophages in this region of the fetal heart, and this function of epicardium depended on its expression of the transcription factor WT1. Thus, tissue-specific cues and transcriptional programs recruit or retain embryonic macrophages in their final abodes, where they help to shape organ homeostasis and injury responses.