Erythroid series

Erythropoietin is an endogenous glycosylated protein hormone that is produced mainly in the kidneys and stimulates the production of members of the erythroid series of blood cells. Epoetin is the name that has been given to recombinant forms, of which there are four epoetin alfa, epoetin beta, epoetin omega, and darbepoetin alfa. Darbepoetin alfa is a supersialylated form of erythropoietin with a longer half-Ufe. 

In cells of the erythroid series (that is, cells which are precursors of erythrocytes and which synthesise haemoglobin) transferrin molecules carrying Fe3+ bind to specific receptor sites on the cell membrane and... 

Differentiation of cells of the erythroid series is controlled by the hormone erythropoietin. Anemia stimulates erythropoietin formation, and this stimulates erythropoiesis. Erythropoietin also increases hemoglobin synthesis by stimulating synthesis of the corresponding messenger RNA (Krantz and Goldwasser, 1965). However, the possible link between erythropoietin and activation of adult hemoglobin (HbA and not HbF) has not yet been studied. 

In other words, repression may vary in its embryologic depth. For example, repression of the fetal hemoglobin system which takes place not long before birth is superficial repression of late type. In the same stem cells of the erythroid series ability to synthesize collagen or antibodies, for example, is repressed more and earlier. Syntheses characteristic of, for example,... 

Repression of fetal hemoglobin synthesis (late repression) in cells of the erythroid series, as we have seen, is not absolute. However, it is nearly absolute in cells of a different specialization, in which it took place earlier. Although in both cases the same gene is repressed, it is undoubtedly repressed differently, by different repressors, by different methods. In cells of the erythroid series the repressor of the HbF gene is connected somehow with the genome activating synthesis of the /5-chain of HbA in leucocytes, for example, it acts in connection with activation of the subprogram of leucocytic specialization. 

In the case of the experiments of Umiel et al., the histological observations of liver cultures are enlightening. The embryonic liver is normally filled with cells of the erythroid series (ef. Barker, 1967), but after 1 day in culture most of the normoblasts either die or transform into erythrocytes. After 3 days, foci of lymphocyte-like cells can be seen, and these cells become more dominant in the next several days. 

During recovery from amidopyrine-induced agranulocytosis there is often a dramatic bone marrow response with the appearance of large numbers of immature granulocytes in the peripheral blood. It may be difficult to differentiate this condition from acute myeloid leukaemia (8 ). In some cases, amidopyrine and related drugs may cause selective depression of the thrombocyte series without involvement of erythroid or myeloid elements (9 ). 

Erythroid toxicity of azathioprine. Quart. J. Med., New Series XLIV, 57. 

In man the synthesis of hemoglobin occurs principally in erythroid cells in the bone marrow. In accordance with the generally accepted Unitarian theory of hematopoiesis, primitive stem cells in the bone marrow give rise to cells of the erythroid, myeloid, and platelet series. In erythropoiesis the stem cell differentiates to form the proerythroblast. Successive mitotic divisions yield basophilic, polychromatophilic, and orthochromatic erythroblasts. Subsequent maturation to normoblasts, reticulocytes, and mature erythrocytes involves no further mitotic division. 

If the yolk sac were to serve as an origin of lymphoid cells, it would be attractive to reconsider the old hypothesis of a single cell precursor of all blood cells of both erythroid and lymphocytic series. Modifications induced by tissue interactions as well as a series of hormonal agents could then account for the divergence of cell lines. 

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