Hematopoiesis is the process by which all the different cell lineages that form the blood and immune system are generated from a common pluripotent stem cell. During the life of an individual, two separate hematopoietic systems exist, both arising during embryonic development but only one persisting in the adult. The primitive hematopoietic system is derived from the extra-embryonic yolk sac and consists mainly of nucleated erythroid cells, which carry oxygen to the developing embryonic tissues. As the embryo increases in size, this early circulatory system is superseded by the more complex definitive hematopoietic system, which originates within the embryo itself and continues throughout adult life.
The definitive hematopoietic system is made up of all adult blood cell types including erythrocytes and cells of the myeloid and lymphoid lineages. All these cells are derived from multipotent hematopoietic stem cells (HSCs) through a succession of precursors with progressively limited potential under the control of specific cytokines such as interleukins and granulocyte/monocyte-stimulating factors. In most cases, the cytokines that determine differentiation of hematopoietic stem cells into a particular lineage are well defined. However, the factors that regulate hematopoietic stem cell generation and maintenance of multipotency remain incompletely understood.