The inner region, or marrow, is one of the largest organs of the body, is located within the bones. It fills the shaft of the long bones, the trabeculae ( spaces within cancellous tissue ), and extends into the boney canals.
Bone marrow may contain fat cells, fluid, fibrous tissue, blood vessels and blood formimg (hematopoietic) cells. Marrow appears yellow in color when it holds many fat cells and red when it has more blood forming material. The marrow is the principle site for blood formation (hematopoiesis), which occurs primarily within the bones of the legs, arms, ribs, breastbone, and spine.
Stem Cells
Many of the blood cells that comprise the bloodstream within the arteries and veins are born and mature within the marrow. They are derived from hematopoietic cells that are called stem cells. Stem cells within the bone marrow continously divide to form new cells.
Some of the new cells remain unchanged as stem cells and have a lifelong capacity for self renewal. These cells are called pluripotential cells. Other unipotential stem cells have a limited capacity for self-renewal also known as progenitor cells. Unipotential cells become committed to forming only one type of blood cell line, eythrocytes (red blood cells), leukocytes (white blood cells) and platelets.
Colonies of progenitor cells provide offspring of increasing differentiation (maturity). They react to specific compounds known as protein. Proteins stimulate the progenitor cells until they transform into the appropriate young blood cell known as a " Blast Cell".
