Bone marrow is found inside the bones, and it functions to produce the body's blood cells through a process called hematopoiesis. Bone marrow comes in two types: red marrow and yellow marrow. Within the red bone marrow, red blood cells, white blood cells and platelets are manufactured. From the bone marrow, they are distributed throughout the body via the circulatory system.
Yellow marrow consists mostly of fat cells. In adults, yellow marrow is found primarily in the narrow middle portion of long bones such as the femur in the legs and humerus in the arms. This type of marrow does not create blood cells, but if the body needs to produce large quantities of blood, the yellow marrow can be transformed into red marrow.
All bone marrow is red when a person is born, but during maturation and aging, a portion of the marrow becomes yellow. In adults, about half of the bone marrow is red. Flat bones such as the skull, scapulae, ribs, pelvis and sternum contain red marrow, as do the rounded ends of the long bones.
Blood cells are produced by the transformation of hematopoietic stem cells, any one of which can become any one of the three types of blood cells. In a single day, the red marrow in the human body will create as many as 100 trillion blood cells. Some diseases such as leukemia are caused by a malfunction in the production of white blood cells and can be treated through a transplant of red marrow.
This marrow is sometimes referred to as myeloid tissue. Myeloid tissue is a particular kind of body tissue that is able to produce blood cells. Although the majority of this tissue is found in the red marrow, it also can be found in the spleen, liver and thymus. In some cases, these other organs will resume their function as haematopoietic organs, often leading to enlargement. During development of a fetus, the liver functions to produce blood cells, so it seems disproportionately large during various stages of development.
Other cells found in the red marrow include mesenchymal stem cells. These cells are found in the stroma, or the non-blood-producing parts of the bone marrow, and can differentiate into many types of cells, from osteoblasts that can produce bone to neurons or nerve cells. Mesemchymal stem cells remain of great interest to scientists who wish to harvest stem cells that can be used for various medicinal purposes such as to treat diabetes or other diseases.