Scavenger macrophages and central macrophages of erythroblastic islands in liver hemopoiesis of the fetal and early postnatal mouse: a semithin light- and electron-microscopic study.

The livers of 12-day mouse embryos contain many hemopoietic cells in the hepatic cords and macrophages which are laden with large heterophagosomes in the sinusoids. Macrophages, as scavengers, engulfed circulating primitive erythroblasts from the yolk sac, as well as nuclei expelled from erythroblasts. At the 14th day of gestation, scavenger macrophages in mitosis were seen, extending long and thin cytoplasmic projections from their cell surface. At the 13-14 day of gestation, macrophages migrated from sinusoids into hepatic cords, and erythroblasts gathered around them, thus forming cell clusters designated as erythroblastic islets. Central macrophages formed a cell-socket-like structure on their cell surface for surrounding erythroblasts, reflecting the close association between macrophages and erythroblasts. In the erythroblastic islands, contained in late fetal and neonatal livers, erythroblasts have dissociated from the central macrophages having a starfish-shaped cell profile. The macrophage-erythroblastic association in the islands became less marked rapidly after birth.