Hemopoietic events in human embryonic spleens at early gestational stages.

The appearance of hemopoietic progenitor cells in human embryonic spleens at early gestational stages was observed with a transmission electron microscope. In spleens of 6- to 7-week-old embryos hemocytoblasts, proerythroblasts and megakarycotyic precursors were identified, located between the mesenchymal cells, suggesting that hemopoiesis in the embryonic spleen is an extravascular process. The relationship between hemopoietic and mesenchymal cells in the embryonic spleen is discussed.