The events of erythropoiesis in cultured embryonic mouse spleens isolated on the 12th day of gestation were compared with those occurring in spleens of corresponding gestational days in vivo. It was found that the development of erythroid precursors in the cultured spleen depended on the presence of erythropoietin in the culture medium. A progressive increase in the number of erythroid precursors, with a marked shift to mature cells, was observed in spleens cultured with erythropoietin. Although the origin of the hemocytoblasts in the spleen isolated in the earliest stage of its development remains obscure, the present study provided evidence that erythropoiesis in the embryonic mouse spleen was indigenous to this organ and did not result from colonization of erythroid precursors originating in the liver.

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