Bone marrow is a collection of soft spongy tissues found in the middle of most bones. It is the fourth largest organ in the body by weight, following bone, muscle, and fat. The bone marrow is responsible for manufacturing all the blood cells although some of these cells are further developed elsewhere in the body.
All cells in the body are made from precursor cells known as stem cells. There are different types of stem cell that mature into different types of cell - many cell types go through several stages in their maturation process.
Both red blood cells (erythrocytes) and white blood cells (leukocytes) are produced from a type of stem or "blast" cell called a "totipotent" cell. This process known as "hematopoiesis".
Totipotent stem cells develop into "multipotent" stem cells which go on to form red blood cells and "lymphoid" stem cells which are the precursors of all white blood cells - the immune system cells.
Lymphoid stem cells further differentiated into various types of leukocyte - neutrophils, eosinophils, basophils, monocytes and lymphocytes.
Lymphocytes diffentiate further into immature T-cells and B-cells in the marrow. The T-cells are released in an antigen independent state and are further matured in a gland called the thymus. Along with another organ, the spleen, bone marrow serves as an antigen-processing environment for B-cells.
Bone marrow Links:
Growth Factors: Hematopoiesis: History and Scientific Background
Blood cells and the CBC
Haematologica on Internet