Three pathways to mature macrophages in the early mouse yolk sac

The existence of macrophages (M Phi) of yolk-sac (YS) origin has been reported in all vertebrate models. However, the nature of their precursors and pathways of differentiation have not been elucidated. Phenotypic and differentiation potential analyses of YS at 7.5 to 10 postcoital days (dpc), performed in CX(3)CR1(GFP) embryos, allowed us to discern 3 independent M Phi populations. A first transient wave consisted of mature, maternal-derived M Phi present as early as 7.5 to 8 dpc. A second wave of committed M Phi precursors arose at 8 dpc (2-4 somite stage) and was followed by a third wave of erythromyeloid precursors (4-6 somite stage). Both types of precursors displayed similar phenotypes and gave rise to CX(3)CR1/green fluorescent protein (GFP)-positive M Phi, but differed by their differentiation potential, at the clonal level. The combined data of phenotypic, gene-expression, and in situ analyses allowed us to conclude that the previously named "primitive M Phi" corresponded to a mixture of the first transient wave and committed M Phi precursors. Both YS-derived precursors followed a developmental pathway common to adult M Phi and could be qualified as definitive.