Haematopoiesis
Haematopoiesis is the formation of blood cellular components. This can occur 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, the blood formation will occur in the spleen, liver and lymph nodes. When bone marrow becomes developed, it will assume 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 of the 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 vertebrate forms, haematopoiesis can occur wherever there is a loose stroma of connective tissue and slow blood supply, such as the gut, spleen, kidney or ovaries.
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