Hematopoietic progenitor cells (HPCs) or hematopoietic stem cells (HSCs) are cells present in blood and bone marrow. HPCs are capable of forming mature blood cells, such as red blood cells (the cells that carry oxygen), platelets (the cells that help stop bleeding) and white blood cells (the cells that fight infections).
HPCs are used in the treatment of many malignant (e.g., leukemia, lymphoma) and non-malignant (e.g., sickle cell disease) diseases to replace or rebuild a patient's hematopoietic system. This type of treatment is called a bone marrow or stem cell transplant. HPCs also have been used in clinical trials with U.S. FDA oversight for the treatment of autoimmune diseases, genetic diseases and other indications.
Hematopoiesis is the continuous, regulated process of renewal, proliferation, differentiation, and maturation of all blood cell lines. These processes result in the formation, development, and specialization of all functional blood cells that are released from the bone marrow into the circulation. Mature blood cells have a limited lifespan (e.g., 120 days for red blood cells [RBCs]) and a cell population capable of self-renewal that sustains the system. A hematopoietic stem cell (HSC) is capable of self-renewal (i.e., replenishment) and directed differentiation into all required cell lineages.1 Thus the hematopoietic system serves as a functional model to study stem cell biology, proliferation, and maturation and their contribution to disease and tissue repair.
Hematopoiesis in the developing human can be characterized as a select distribution of embryonic cells in specific sites that rapidly changes during development.2 In contrast, hematopoiesis in healthy adults is restricted primarily to the bone marrow. During fetal development, the restricted, sequential distribution of cells is initiated in the yolk sac and then progresses in the aorta-gonad-mesonephros (AGM) region (mesoblastic phase), then to the fetal liver (hepatic phase), and finally resides in the bone marrow (medullary phase). Because of the different locations and resulting microenvironmental conditions (i.e., niches) encountered, each of these locations has distinct but related populations of cells.