Erythroblast (hemoglobin shows pink in cytoplasm, synthesizes hemoglobin, nucleus becomes small to accomodate the hemoglobin)
Normoblast (small nucleus, no basophilic cytoplasm is evident, appears pink in color)
Reticulocyte (no nucleus, 80% of hemoglobin of mature RBC, hemoglobin synthesis continues do to polyribosomes)
Erythrocyte (reticulocyte without polyribosomes & mitochondria, slighlty smaller with pink cytoplasm)
Controlled by a negative feeback system using Erythropoietin which is secreted mainly by the kidneys (manufactured by the liver to a lesser degree). Erythropoietin is released in repsonse to low O2. Circulates in the blood to bone marrow, causing the marrow to stimulate hemocytoblasts to differentiate. Levels of Erythropoetin will decrease as O2 rises.