Hematopoiesis in the Embryonic Mouse Spleen: An Electron Microscopic Study

The events of hematopoiesis in the C57/BI6J embryonic mouse spleen are described, beginning with the appearance of the splenic rudiment (12th gestational day). Immature erythroid precursors appear on day 12 of embryonic development, but foci of mature red cell precursors were not seen until day 15. Red cell precursors were observed in the spleen up to day 19 after birth. Mature polymorphonuclear leukocytes were present by day 12, whereas megakaryocytic precursors appear on day 12. Although young cells of the lymphocytic series were identified on the 16th gestational day, a marked proliferation of lymphatic cells was noted at the end of the gestational period and immediately after birth.