Abstract
Primitive macrophages first appear in the blood islands of the mouse yolk sac on the ninth day of gestation. After the tenth day of fetal life, these cells differentiate into fetal macrophages and become mature, with the development of intracellular organelles. They appear in the mesenchymal layer and further immigrate into the extraembryonic coelom. The fetal macrophages do not show any cytochemical peroxidase or 5'-nucleotidase activity, and they possess a marked proliferative capacity. Promonocytes or monocytes that have an incomplete ultrastructure emerge in the blood islands of the yolk sac a day after the occurrence of the fetal macrophages. These events suggest that fetal macrophages differentiate from primitive macrophages before the development of promonocytes or monocytes in the mouse yolk sac; they actively proliferate and are colonized into the embryonic tissues. These results also indicate that the ontogeny of the monocyte/macrophage is different in the early embryo compared with its later developmental stages.
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