ABSTRACT
The hematopoietic system generates mature, functional progeny of eight major lineages of blood cells. It is maintained from a relatively small, heterogeneous pool of stem cells which retain a high capacity for self-renewal throughout the life of the animal. Stem cells generate expanded numbers of more mature progeny, hematopoietic progenitor cells, which have a limited capacity for self-generation and are generally restricted in their capacity to differentiate into one or a limited number of hematopoietic cell types. Progenitor cells have a high proliferative potential and may generate many thousands of maturing progeny which develop the typical morphological characteristics of functional, end-stage blood cells. Hematopoiesis is established early in embryonic development within the blood islands of the yolk sac. Definitive, or adult hematopoiesis is first located
in the fetal liver and then later shifts to the spleen and ultimately the bone marrow, forming the hematopoietic cell populations that sustain blood cell formation throughout life.
