Bone marrow is soft, gelatinous tissue that fills the medullary cavities, the centers of bones. The two types of bone marrow are red bone marrow, known as myeloid tissue, and yellow bone marrow, or fatty tissue
Both types of bone marrow are enriched with blood vessels and capillaries.
Bone marrow makes more than 200 billion new blood cells every day. Most blood cells in the body develop from cells in the bone marrow.
Bone marrow stem cells
The 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, also known as marrow stromal cells. These produce fat, cartilage, and bone.4
Stem cells are immature cells that can turn into a number of different types of cell.
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 megakaryocytes or platelets, as well as T cells, B cells, and natural killer cells.
The different types of hematopoietic stem cells vary in their regenerative capacity and potency.
Some are multipotent, oligopotent or unipotent as determined by how many types of cell they can create.
Pluripotent hematopoietic stem cells have the following properties:
- Renewal: They can reproduce another cell identical to themselves.
- Differentiation: They can generate one or more subsets of more mature cells.
The process of development of different blood cells from these pluripotent stem cells is known as hematopoiesis.
It is these stem cells that are needed in bone marrow transplant.
Stem cells constantly divide and produce new cells. Some new cells remain as stem cells and 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 100-120 days for red blood cells. They are constantly being replaced. The production of healthy stem cells is vital.
The blood vessels act as a barrier to prevent immature blood cells from leaving the 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. These may be harvested from peripheral, or circulating, blood.
The blood-forming stem cells in red bone marrow can multiply and mature into three significant types of blood cells, each with their own job:
- Red blood cells (erythrocytes) transport oxygen around the body
- White blood cells (leukocytes) help fight infection and disease. White blood cells include lymphocytes - the cornerstone of the immune system - and myeloid cells which include granulocytes: neutrophils, monocytes, eosinophils, and basophils
- Platelets (thrombocytes) help with clotting after injury. Platelets are fragments of the cytoplasm of megakaryocytes, another bone marrow cell.
Once mature, these blood cells move from the marrow into the bloodstream, where they perform important functions required to keep the body alive and healthy.
Mesenchymal stem cells are found in the bone marrow cavity. They differentiate into a number of stromal lineages, such as:
- chondrocytes (cartilage generation)
- osteoblasts (bone formation)
- adipocytes (adipose tissue)
- myocytes (muscle)
- endothelial cells