Haematopoiesis
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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 lymphocytes and monocytes.
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.