Normally immature blood cells in the bone marrow of adults do not appear in the peripheral blood, but they are visible in pathological conditions. They are also occasionally seen in newborns. Bone marrow hematopoietic cells can be differentiated into primitive erythroid, granulocytic, and macrophage lineages, and these blood cells are differentiated and matured in the bone marrow and released into the peripheral blood as red blood cells, leukocytes, and platelets, so that generally the peripheral blood is full of mature blood cells. In pathological conditions, such as severe anemia, reticulocyte hyperplasia may be seen in the peripheral blood, in addition to hematologic disorders such as myelodysplastic syndromes and leukemia, a large number of bone marrow naïve cells are seen in the peripheral blood.