Medical Definition of Hematopoiesis Hematopoiesis: The production of all types of blood cells including formation, development, and differentiation of blood cells. Prenatally, hematopoiesis occurs in the yolk sack, then in the liver, and lastly in the bone marrow. In the normal situation, hematopoiesis in adults occurs in the bone marrow and lymphatic tissues. All types of blood cells are derived from primitive cells that are pluripotent (they have the potential to develop into all types of blood cells).