Monday, October 09, 2006

Haematopoiesis

Haematopoiesis is the formation of blood cellular components. All of the cellular components of the blood are derived from haematopoietic stem cells. The term pluripotent refers to the ability of a cell to become many different types of cell. Pluripotent haematopoetic cells can become any type of cell in the blood system. The pluripotent cells determine what type of cell to become, or differentiate, in a step-wise fashion.

In developing embryos, blood formation occurs in aggregates of blood cells in the yolk sac, called blood islands. As development progresses, blood formation occurs in the spleen, liver and lymph nodes. When bone marrow develops, it eventually assumes the task of forming most of the blood cells for the entire organism. However; maturation, activation, and some proliferation of lymphoid cells occurs in secondary lymphoid organs . While most haematopoiesis in adults occurs in the marrow of the long bones such as the femurs, it also occurs in spongy bone. In some cases, the liver, thymus, and spleen may resume their haematopoietic function if necessary.