From the functional point of view the processes of differentiation during the formation of the red blood-cells are characterized by the synthesis of haemoglobin.
If the haemoglobin content of single erythroblasts is measured one finds that most of the formation of haemoglobin occurs in the later stages of erythropoiesis (Table 1).
When it reaches these stages of development the growth and division processes of the erythroblasts have been completed. The cytoplasm has lost its asophilia and the nuclei are becoming pycnotic. The cells are, however, capable of forming haemoglobin.
From the general point of view of cell physiology, it seems reasonable to believe that only the prosthetic group (haem) is being synthesized during these later stages of erythropoiesis. The globin-protein might be manufactured mainly in earlier, growing erythroblasts in the presence of ribosepolynucleotides which are abundant in the cytoplasm of these cells.