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Bone marrow is soft, gelatinous tissue that fills the medullary cavities, or the centers of bones. The two types of bone marrow are red bone marrow, known as myeloid tissue, and yellow bone marrow, known as fatty tissue.
Both types of bone marrow are enriched with blood vessels and capillaries.
Bone marrow makes more than 220 billion new blood cells every day. Most blood cells in the body develop from cells in the bone marrow.
Bone marrow stem cells
Bone marrow contains two types of stem cells: mesenchymal and hematopoietic.
Red bone marrow consists of a delicate, highly vascular fibrous tissue containing hematopoietic stem cells. These are blood-forming stem cells.
Yellow bone marrow contains mesenchymal stem cells, or marrow stromal cells. These produce fat, cartilage, and bone.
Stem cells are immature cells that can turn into a number of different types of cells.
Hematopoietic stem cells in the bone marrow give rise to two main types of cells: myeloid and lymphoid lineages. These include monocytes, macrophages, neutrophils, basophils, eosinophils, erythrocytes, dendritic cells, and megakaryocytic, or platelets, as well as T cells, B cells, and natural killer (NK) cells.
The different types of hematopoietic stem cells vary in their regenerative capacity and potency. They can be multipotent, oligopotent, or unipotent, depending on how many types of cells they can create.
Pluripotent hematopoietic stem cells have renewal and differentiation properties. They can reproduce another cell identical to themselves, and they can generate one or more subsets of more mature cells.
The process of developing different blood cells from these pluripotent stem cells is known as hematopoiesis. It is these stem cells that are needed in bone marrow transplants.
Stem cells constantly divide and produce new cells. Some new cells remain as stem cells, while others go through a series of maturing stages, as precursor or blast cells, before becoming formed, or mature, blood cells. Stem cells rapidly multiply to make millions of blood cells each day.
Blood cells have a limited life span. This is around 120 days for red blood cells. The body is constantly replacing them. The production of healthy stem cells is vital.
The blood vessels act as a barrier to prevent immature blood cells from leaving bone marrow.
Only mature blood cells contain the membrane proteins required to attach to and pass through the blood vessel endothelium. Hematopoietic stem cells can cross the bone marrow barrier, however. Healthcare professionals may harvest these from peripheral, or circulating, blood.
