Marrow is a soft, netlike mass of connective tissue found in the medullary cavities of long bones, in the irregular spaces of spongy bone, and in the larger haversian canals of bone tissue.
There are two kinds of marrow—red and yellow.
Red marrow functions in the formation of red blood cells (erythrocytes), certain white blood cells (leukocytes), and blood platelets (thrombocytes). It is red because of the red, oxygen-carrying pigment called hemoglobin, which is contained within the red blood cells it produces. Red marrow occupies the cavities of most bones in a newborn child, but with age more and more of it is replaced by yellowish, fat-storage cells that are inactive in blood cell production.
In an adult, red marrow is found primarily in the spongy parts of the bones of the skull, ribs, sternum, clavicles, vertebrae, and pelvis. If the blood cell supply is deficient, yellow marrow may change back into red marrow and become active in blood cell production.
Red marrow may be damaged or destroyed by excessive exposure to radiation, adverse drug reactions, or the presence of cancerous tissues. The treatment of this condition sometimes involves a bone marrow transplant.
In this procedure, normal red marrow cells are removed from the spongy bone of a donor by means of a hollow needle and syringe. These cells are then injected into the recipient's blood with the hope that they will lodge in the bone spaces normally inhabited by red marrow and will, in time, replace the damaged tissue
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