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About 40 years ago it was recognized that a humoral factor, later identified as erythropoietin (Epo), was able to regulate the response of bone marrow to changes in red cell mass.1,2 It was also found that Epo was produced by the kidney in response to renal tissue hypoxia,3,4 and a quantitative assay based on the measurement of newly produced red cells labeled with59Fe was developed.2,5 The introduction of assays that allow cells to proliferate and differentiate in culture has led to the conclusion that Epo primarily acts on cells morphologically unrecognizable as erytroblasts. Semisolid cultures using methylcellulose, plasma-clot, and agar6–9 showed that two classes of erythroid progenitors can be observed at different times of incubation.7,8,10 Human bone marrow mononuclear cells in culture give rise to erythroid colonies after 4 to 5 days of incubation,11 while a longer (8–9 days) incubation time determines the appearance of larger colonies,7 and even larger and more hemoglobinized ones appear after 14 days. The first type is currently defined as colony-forming unit-erythroid (CFU-E)8; 14 and 8- to 9-day colonies are referred to as burst-forming unit-erythroid (BFU-E)12 and intermediate BFU-E,7 respectively (Fig. 3.1). Murine counterparts of these cells grow after 2 (CFU-E), 3 to 4 (intermediate BFU-E), and 8 (BFU-E) days.13 In addition, a murine progenitor with higher proliferative and differentiative capacity is seen after 9 to 12 days of incubation.7 |
