Brain Macrophages: Close or Distant Relatives?

The concept of a mononuclear phagocyte system in which tissue-resident macrophages arise from the circulating monocytes has been dogma in immunology for more than half a century [1]. While monocytes do give rise to macrophages under certain conditions, recent studies clearly demonstrate that monocytes do not primarily contribute to most adult tissue-resident macrophage pools under the steady state [2]. Instead, the majority of adult tissue-resident macrophages are derived from erythromyeloid progenitors （EMPs） in the yolk sac that seed tissue before birth [2].