Embryonic origin of the mouse macrophage.

Progenitor cells capable of giving rise to functional macrophages in in vitro culture were first detectable in the fetal yolk sac of the mouse between day 7 and 8 of gestation. Macrophage progenitors were not detectable outside the yolk sac until day 11. Although the early yolk sac contained macrophage precursors, no cells with the morphologic or functional characteristics of mouse promonocytes or more mature macrophages were observed. Promonocytes and macrophages were first identified in the 10-day yolk sac and 11-day fetal liver. These cells were characterized by surface receptors for lgG immunoglobulin, peroxidase activity (promonocytes), glass adherence, and phagocytosis of a large yeast particle (macrophages). From these observations, we conclude that the early fetal yolk sac is the embryonic site of origin of the macrophage precursor and that this precursor is “proximal” to the promonocyte on the pathway of sequential macrophage maturation.