Polychromatophilic erythroblast

The synthesis of mRNA terminates in the polychromatophilic erythroblast. Thus, no RNA is synthesized in reticulocytes and orthochromic erythroblasts. The mRNA synthesized in the basophilic erythroblast is stable and remains in the cell for long periods. This mRNA is active even in the reticulocyte stage. Protein synthesis starts after the termination of RNA synthesis in the polychromatophilic erythroblast, The rate of hemoglobin synthesis is maximal in polychromatophilic and orthochromic erythroblasts (Marx and Kovach, 1966). There is a correlation between the number of polysomes and the rate of hemoglobin synthesis (Table 40). This was demonstrated with the aid of electron microscopy and direct visualization of polysomes and separate ribosomes. 

Late erythroblast (Polychromatophilic normoblast rubricyte) Smaller, coarse chromatin condensation 0 More abundant, patchy, irregular, acidophilic-basophilic 3-10... 

Thus, in 160 hours, one daughter of the stem cell has yielded 16 erythrocytes. The other daughter also divides after 20 hours. Thus two descendents differentiate and divide. But after the second generation of cells, there are alternative pathways different from the regular erythropoiesis. Two of the four basophilic erythroblasts divide and yield four polychromatophils that hasten to differentiate into normoblasts without further division. Two other basophilic erythroblasts enter a physiological cul-de-sac referred to as ineffective erythropoieses. On the basis of this model, one stem cell yields 20 erythrocytes in a period of 130 hours. 

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. 

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