What is Bone Marrow?

Bone marrow is soft, gelatinous tissue that fills the medullary cavities, the centers of bones. It is a spongy tissue inside some of the bones in the body, including the hip and thigh bones. Bone marrow contains immature cells, called stem cells.

Many people with blood cancers, such as leukemia and lymphoma, sickle cell anemia, and other life-threatening diseases, rely on bone marrow or cord blood transplants to survive.

Healthy bone marrow and blood cells are needed in order to live. When disease affects bone marrow so that it can no longer function effectively, a marrow or cord blood transplant could be the best treatment option; for some patients it is the only potential cure. 

The two types of bone marrow are red bone marrow, known as myeloid tissue, and yellow bone marrow, or fatty tissue.

1. Red bone marrow helps produce blood cells
   
   
2. Yellow bone marrow helps store fat.

Read on to learn more about different functions of red and yellow bone marrow as well as the conditions that affect bone marrow.

Function of Red Bone Marrow

Red bone marrow is involved in hematopoiesis. This is another name for blood cell production. Hematopoietic stem cells that are found in red bone marrow can develop into a variety of different blood cells, including:

Red blood cells. These are the cells that work to carry oxygen-rich blood to the cells of the body. Old red blood cells can also be broken down in red bone marrow, but this task is mostly performed in the liver and spleen.

Platelets. Platelets help your blood clot. This prevents uncontrolled bleeding.

White blood cells. There are several types of white blood cells. They all work to help your body fight off infections.

Newly produced blood cells enter your bloodstream through vessels called sinusoids.

As you age, your red bone marrow is gradually replaced with yellow bone marrow. And by adulthood, red bone marrow can be found only in a handful of bones, including the:

1. skull
2. vertebrae
3. sternum
4. ribs
5. the ends of the humerus (upper arm bone)
6. pelvis
7. the ends of the femur (thigh bone)
8. the ends of the tibia (shin bone)

Functions of Yellow Bone Marrow

Yellow bone marrow is involved in the storage of fats. The fats in yellow bone marrow are stored in cells called adipocytes. This fat can be used as an energy source as needed.

Yellow bone marrow also contains mesenchymal stem cells. These are cells that can develop into bone, fat, cartilage, or muscle cells.

Remember, over time, yellow bone marrow starts to replace red bone marrow. So, most bones in an adult body contain yellow bone marrow.

Bone Marrow Stem Cell 

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.12

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:

1. Red blood cells (erythrocytes) transport oxygen around the body
   
   
2. 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
   
   
3. Platelets (thrombocytes) help with clotting after injury. Platelets are fragments of the cytoplasm of megakaryocytes, another bone marrow cell.
   
   
4. 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:

1. Chondrocytes (cartilage generation)
2. Osteoblasts (bone formation)
3. Osteoclasts
4. Adipocytes (adipose tissue)
5. Myocytes (muscle)
6. Macrophages
7. Endothelial cells
8. Fibroblasts