BONE MARROW -
Noun: The growth and development of normal cells are carefully controlled in the bone marrow to produce the correct numbers of each type of blood cell to keep the body healthy. Although, there are many different types of blood cells, all cells made in the bone marrow start as a single kind of cell called a stem cell . Stem cells make up only a very small proportion of the cells in the bone marrow.

This is a picture of normal bone marrow at medium magnification. About one half of the marrow is filled with red blood cells, white blood cells, platelets,and the cells which produce them. The large white cells are fat cells called steatocytes .

The bone marrow stores stem cells until the body needs a specific type of mature blood cell. Then, by using the stem cells in reserve, the bone marrow can rapidly produce many red cells, white cells or platelets. As stem cells mature, their features become more and more distinct until the stem cells develop into a specific type of blood cell. See Haematopoiesis for a picture of the process.

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