Ultrastructural study of the erythropoietic events in human embryonic livers.

The ultrastructural features of the erythropoietic events in 5- to 9-week embryonic livers are described. By the 5th week of gestation, the human embryonic liver becomes an active site of erythropoiesis represented by the formation of reticular-mesenchyme cells and hemocytoblasts. At the 6--7th week, the first proerythroblasts and islands of polychromatophilic and orthochromatic erythroblasts can be detected. From this stage, by the 8th week of embryonic development, the liver assumes its function as a fully developed embryonic organ. In the embryonic livers studied there were no signs of granuloor lymphopoiesis. Although megakaryocytes were found in the 8-week embryonic liver, there is no evidence that these cells are produced in the liver. Macrophages were found in the 5-week embryonic liver.