Hematopoietic stem cells (HSCs) are multipotent stem cells defined by their ability to self- renewal, differentiation and maintenance of all blood cell types in the hematological system during the entire lifetime of the organism. This
physiological process, called hematopoiesis, is controlled by several complex interactions between genetic processes in blood progenitor cells and bone marrow microenvironment. On the other hand, we have some subjects as follows: first, in bone marrow at birth, the total marrow space is occupied by active hematopoietic marrow. As body growth progresses and marrow space increases during infancy up to youth, only part of that space is needed for hematopoiesis. Second, in peripheral blood differential leukocyte count the normal values of lymphocytes have difference in children and adults. Third, the mechanism(s) by fusion genes like BCR-ABL1 promotes the transition from benign state to the fully malignant one is still unclear.Messenger RNA for BCR-ABL1 can occasionally be detected in normal individuals. As we know, acute lymphoblastic leukemia (ALL) is a malignant neoplasm of lymphoblasts characterized by the clonal accumulation of immature blood cells in the bone marrow. These abnormal cells are arrested in the lymphoid stage of the normal maturation pathway. Aberrations in proliferation and differentiation of these cells are common, and normal hematopoiesis is suppressed. Regarding this point, we know the overall cure rate in children is 90%, while about 50% of adults are long-term disease-free survivors. Thus, we should be a better understanding in the role of hematopoietic microenvironment and HSC niches in hemostasis especially in rebuilt or repair of bone marrow after treatment in childhood ALL.