Final answer:
Reticulocytes are the stage at which the immature red blood cells (erythroblasts) are released into the bloodstream. They then mature into erythrocytes, which circulate for approximately 120 days.
Step-by-step explanation:
Erythrocytes, or red blood cells, have a well-defined development process in the bone marrow. The maturation involves several stages, starting from a hematopoietic stem cell differentiating into an erythroblast. As these erythroblasts mature, they go through a phase where they extrude their nucleus and become reticulocytes.
Reticulocytes are the stage at which red blood cells are released into the bloodstream. Once in circulation, these reticulocytes then fully mature into erythrocytes, which are anucleate cells rich in hemoglobin, capable of transporting respiratory gases. This maturation process takes about 1 to 2 days, after which the mature erythrocytes circulate for an average of 120 days before they are destroyed by macrophages and their components are recycled.