Acidophilic erythroblast expels the nucleus and becomes the reticulocyte. As a rule, this happens when erythroblasts pass through pores of the endothelium of the venous sinuses in the bone marrow. Through these 1 - 4 µm pores the erythroblast’s cytoplasm may pass but not the nucleus. Bone marrow macrophages phagocytize and digest the expelled nucleus. Loosing its nucleus, acidophilic erythroblast becomes the reticulocyte and stays in the bone marrow for a few more days. Although there is no nucleus in the reticulocyte, ribosomes, mitochondria and Golgi’s complex are present during this time and the synthesis of hemoglobin is active. RNA-synthesizing ribosomes, present in reticulocytes, are stained with brilliant-cresyl blue as a more or less blue reticulum, the so-called substantia reticulofilamentosa. On its membrane, the reticulocyte has receptors for transferrin so that iron can enter the cell. The presence of transferrin on the membrane of reticulocyte is one of the reasons why reticulocytes are more “sticky” than mature erythrocytes, and why they pass with difficulty from the bone marrow to the peripheral blood. The tricarboxylic acids cycle still exists, allowing the production of larger quantities of ATP than in the mature erythrocyte. When it loses ribosomes, mitochondria, Golgi’s complex (components of the cytoplasm), and transferrin receptors, the reticulocyte decreases in size and becomes the mature erythrocyte.