Haematopoiesis
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Haematopoiesis is the formation of blood cellular components. This process occurs in myeloid tissue, which is found in the bone marrow and lymphatic tissue, such as lymph nodes or the spleen. All of the cellular components of the blood are derived from haematopoietic stem cells.
In developing embryos, blood formation occurs in aggregates of blood cells, called blood islands. As development progresses, blood formation occurs in the spleen, liver and lymph nodes. When bone marrow develops, it assumes the task of forming most of the erythrocytes and granulocytes. Lymphoid organs continue to produce most of the mature lymphocytes and monocytes, though lymphoid progenitor cells originate in the bone marrow. While most haematopoiesis in adults occurs in the bone marrow of the femurs (also the ribs and sternum), the liver, thymus, and spleen may resume their haematopoietic function if necessary (called extramedullary haematopoiesis).
In some vertebrates, haematopoiesis can occur wherever there is a loose stroma of connective tissue and slow blood supply, such as the gut, spleen, kidney or ovaries.
Embryologically, in roughly the 3rd-4th week of gestation the blood precursor cells arises from the yolk sac mesoderm. In esccense the first hematopoetic organ is the yolk sac, then the liver and spleen will take over in week 5 as a temporary but major hematopoietic tissues. Skipping over to about the 5th month of gestation, the bone marrow then becomes increasingly important as the hematopoietic organ. After birth, the blood cells are derived from stem cells of the bone marrow. The bone marrow also produces T-cells that migrate to the thymus where it becomes immunocompentent in the paracortex. Most cells mature in the bone marrow(become immunocompentent).
The main site for hematopoiesis is the bone marrow (after birth). Under normal conditions, the production of blood cells can adjust rapidly to the need of the body, increasing several-fold in a short time. The bone marrow is found in the meduallary canals of long bones and the cavities of cancellous bones. The first cavity to form is in the clavicle--this becomes clinically. Of clinical importance is the sternum when diagnosing certain types of Leukemia.
Taken from Dr. Yin's Notes at Ross University School of Medicine 11/08/2005.