Pathways for delivery of iron and heme iron to developing erythroblasts. Transferrin (Tf) is the major source of iron (red circles) as it emerges from the plasma to bind transferrin receptors (TfRs), becomes internalized, and releases iron into acidic vesicles. Central nurse macrophages found in erythroblastic islands may deliver iron to developing erythroid cells, and secreted ferritin (Ftn) has been observed in the space between nurse macrophages and erythroblasts. Because both developing erythroid cells and macrophages express plasma membrane heme transporters such as Hrg1 and Flvcr1, it is possible that heme (red crosses) itself is transferred to developing red blood cells.