Sites of Hematopoiesis

  

   Hematopoiesis/Hemopoiesis is the process by whitch blood cells are formed. 

Sites of Hematopoiesis

Yolk sac

     From the 18^th day after fertilization, the yolk sac begins hematopoiesis. The cells made here are erythrocytes & few macrophages.

Aorta-gonads-mesonephros (AGM) region

     Located along the developing aorta. This region has the ability to make a wider range of hematopoietic cells including lymphocytes.

    At about the 3^rd month of fetal life, the yolk sac & AGM discontinue their role in hematopoiesis.

Liver

    At about the 3^rd month of fetal life, the liver becomes the chief site of blood cell production. The liver continues to produce a high proportion of erythroid cells, but myeloid and lymphoid cells begin to appear in greater numbers.

     As fetal development progresses, hematopoiesis also begins to a lesser degree in the spleen, kidney, thymus, and lymph nodes. Erythroid and myeloid cell production as well as early B cell (lymphocyte) development gradually shifts from these sites to bone marrow during late fetal and neonatal life as the hollow cavities within the bones begin to form.

Bone Marrow

     The bone marrow becomes the primary site of hematopoiesis at about the 6^th month of gestation and continues as the primary source of blood production after birth and throughout adult life.

    The thymus becomes the major site of T cell (lymphocyte) production during fetal development and continues to be active throughout the neonatal period and childhood until puberty. 

     Lymph nodes and spleen continue as an important site of late B cell differentiation throughout life.

NOTE 

     Liver & spleen may return to hematopoiesis after birth if necessary in a  process  called extramedullary hematopoiesis (production of blood cells outside the bone marrow).