Human Anatomy - Kotsan I. Y. 2009

Organs of Hematopoiesis and the Immune System

Hematopoiesis, or hemopoiesis (from the Greek haema meaning Blood and poiesis meaning to make or produce), is The process of blood Cell Formation. In various Multicellular animals, it takes place in different Organs: Blood Vessels (in most invertebrates and some lower vertebrates); Liver and Spleen (in many fish and urodele amphibians); Kidneys (in some fish); intestine (in some fish); and Bone Marrow (in anuran amphibians, reptiles, birds, and mammals).

In humans after birth, Cytology/practical/86.html">Red bone marrow (medulla ossium rubra) is virtually the sole site of hematopoiesis. It produces stem Cells that serve as precursors for all blood and immune system cells. Through complex transformations involving multiple cell divisions and differentiation along three distinct pathways (erythropoiesis, granulocytopoiesis, and thrombopoiesis), progenitor cells ultimately develop into formed elements of the blood—erythrocytes, leukocytes, and platelets—which then enter the bloodstream. Immune system cells, namely lymphocytes, develop from stem cells directly within the bone marrow and the Thymus. From these central Organs of Immunogenesis, lymphocytes migrate via the bloodstream to peripheral immune organs to perform their primary function of defending the body against genetically foreign substances (immune function).