Haematopoiesis is a complex process in which haematopoietic stem cells, the most immature elements of the haematopoietic hierarchy, proliferate and differentiate into various classes of haematopoietic progenitor cells. These progenitor cells have been shown to be able to differentiate into mature blood cells: erythrocytes, lymphocytes, thrombocytes, granulocytes, and monocytes. The pronephros, or head kidney, is a basic organ forming the blood elements, and is also a reservoir of blood cells. Basic haematopoietic structures and mechanisms in fish are similar to those functioning in other vertebrates, and all haematopoietic cell types are very similar to those of mammals.
