Myeloid, or blood cell-forming tissue, is found in the bone marrow and provides the stem cells that develop into erythrocytes, granulocytes, agranulocytes, and platelets. Red marrow is characterized by active hemato-poiesis; yellow bone marrow is inactive and contains mostly fat cells. In the human adult, hematopoiesis takes place in the mar row of the flat bones of the skull, ribs and sternum, the vertebral column, the pelvis, and the proximal ends of some long bones. Erythropoiesis is the process of RBC formation. Bone marrow stem cells (colony-forming units, CFUs) differentiate into proerythroblasts under the influence of the glycoprotein erythropoietin, which is produced by the kidney.

Proerythroblast is a large basophilic cell containing a large spherical euchromatic nucleus with prominent nucleoli.

Basophilic erythroblast is a strongly basophilic cell with nucleus that comprises approximately 75 % of its mass. Numerous cytoplasmic polyribosomes, condensed chro-matin, no visible nucleoli, and continued hemoglobin synthesis characteristics of this cell.

Polychromatophilic erythroblast is the last cell in this line undergoes mitotic divisions. Its nucleus comprises approximately 50 % of its mass and contains condensed chroma-tin which appears in a «checkerboard» pattern. The po-lychnsia of the cytoplasm is due to the increased quantity of acidophilic hemoglobin combined with the basophilia of cytoplasmic polyribosomes.

Normoblast (orthochromatophilic erythroblast) is a cell with a small heterochromatic nucleus that comprises approximately 25 % of its mass. It contains acidophilic cytoplasm because the large amount of hemoglobin and degenerating organelles. The pyknotic nucleus, which is no longer capable of division, is extruded from the cell.

Reticulocyte (polychromatophilic erythrocyte) is an immature acidophilic denucleated RBC, which still contains some ribosomes and mitochondria involved in the synthesis of a small quantity of hemoglobin. Approximately 1 % of the circulating RBCs are reticulocytes.