Erythropoiesis is the formation of red blood cells in hematopoietic tissue, such as the bone marrow or spleen. All maturation stages have a round nucleus and a non-granular cytoplasm. In the process of differentiation, the nucleus becomes pycnotic (dense and small) and finally extruded (the transformation of a normoblast into a reticulocyte). The initial basophilic cytoplasm matures and becomes acidophilic because of hemoglobin formation. In the bone marrow film, the ratio of myelopoietic to erythropoietic cells (M:E ratio) is 3-4 to 1. Daily hematopoiesis in a person who weighs 70 kg amounts to approximately 250-300 x 109 erythrocytes or 3-4 x 109/kg. This output, however, can be increased 10-fold if required.
Pronormoblast --> basophilic normoblast --> polychromatophilic normoblast --> orthochromatic normoblast --> reticulocyte --> erythrocyte
The stages of development up to the polychromatophilic normoblast stage correspond to the mitotic pool of erythropoiesis; the subsequent stages constitute the postmitotic pool.