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Studies on hematopoietic stem cells: XI. Lack of erythroid burst‐forming units (BFU‐E) in patients with aplastic anemia

The marrow concentration of erythropoietic precursors was examined in normal donors and patients with idiopathic aplastic anemia using a plasma clot culture system. On time course observations the heterogeneity of human erythroid precursors assayable in culture was demonstrated. To evaluate human erythropoiesis in vitro, the benzidine‐positive colonies were divided into three groups: small colony, containing 8–50 cells; medium‐sized colony, containing 50–500 cells; and large colony, containing more than 500 cells. The majority of the large colonies assumed the morphology of erythropoietic bursts (BFU‐E) consisted of several subcolonies. The small colonies were counted as CFU‐E1, the medium‐sized as CFU‐E2, and the large as BFU‐E to evaluate the erythroid precursor cell compartment in aplastic anemia.