Stages of RBC`s production

This picture which shows the exact conversion process starting from the hemoblast & ending up with the erythrocyte. click for enlargment

The Erythrocyte production or "Erythropoiesis" begins when a hemocytoblast descendant called a Myeloid Stem Cell is transformed into proerythroblast, which in turn gives arise to the early (basophilic) erythroblasts that produce huge numbers of ribosomes, During these first 2 phases, the cells divide many times.
The Hemoglobin synthesis & iron accumulation occurs as the early erythroblast is transformed into a Late Erythroblast. The color of the cell cytoblasm changes as the Blue-Staining Ribosomes becomes masked by the pink color of Hemoglobin, In this stage & when the last cell produced "Normoblast" accumulates hemoglobin in a concentration of around 34%, most of its organelles are ejected, In addition, it’s nuclear functions ends & is pinched off. This will lead to cell collapsion "To Inward Direction" & will take at the end the bioconcave shape, so the result after assuming the bioconcave shape will be the what is called "The Reticulocyte" , those reticulocytes are considered to be the young erythrocytes which are holding that name because they still contain a network of clumped ribosomes & rough endoplasmic reticulum.
All the entire process from forming the hemocytoblast untill forming the reticulocyte will take around 3 to 5 days, The filled reticulocytes almost are filled to bursting with hemoglobin, then those reticulocytes filled with hemoglobin will enter the blood stream to begin their functions & tasks in oxygen transport. Usually they become fully matured eryhthrocytes whithin 2 days of the release of the reticulocytes, as the reticulocytes ribosomes are degraded by intracellular enzymes.