What Is The Bone Marrow, And What Does It Actually Do?

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What Is The Bone Marrow?

Bone marrow is an encapsulated bundle of stem cells that lies in the centre of certain bones. There are two things that are important to know about bone marrow. The first is that it is located in holes within the bones called marrow cavities. The second is that there are two types of bone marrow. Red bone marrow is found primarily within flat bones like the sternum and is the main focus of this article. Yellow bone marrow is located in the centre of long bones like the femur, often surrounded by a layer of red marrow. 

Red bone marrow makes blood cells and stores blood platelets, while yellow bone marrow stores fat and produces cartilage, fat and bone. The average adult has around 3-4 litres of bone marrow inside their bones, depending on age and gender. The bone marrow is a vital organ for our bodies, because it produces over 30 billion red blood cells and 20 billion white blood cells that circulate through our system each day.

Where Is The Bone Marrow Located?

Yellow bone marrow is located in the medullary cavity, which is enclosed within the endosteum: the inner bone surface consisting of endothelial cells and fibroblasts.

Red bone marrow is found in the cavities of the spongy bone, and also in the outer layers of the medullary cavity, surrounding the yellow bone marrow.

What Cells Are Made In The Bone Marrow?

The red blood cells are the most numerous of all of the blood cells. They transport oxygen throughout our body. The white blood cells include monocytes and granulocytes, which mediate the inflammatory response, and lymphocytes (T cells, B cells and natural killer cells) which are the key components of our adaptive immune system. The platelets help clotting, healing and recovery when we are injured. 

A red blood cell can only live about 120 days before it dies and is replaced by a new one that is made in the bone marrow. A white blood cell can live up to 150 days before it dies and is replaced by a new one, though some lymphocytes may live for years if they become a memory cell. A platelet can live from 5-9 days before it dies and is replaced by a new one.

How Do Blood Cells Leave The Bone Marrow?

While we sometimes view the skeleton as a simple scaffold to support our weight, bone is still a living, growing tissue that must be supplied with nutrients by a network of blood vessels. As blood cells mature, they move towards the central region of the bone marrow, where they exit through a dense network of capillaries (small blood vessels) and enter the bloodstream. 

Prior to leaving the bone marrow, red blood cells eject their DNA-containing nucleus and destroy their organelles, but still retain a residual network of ribosomal RNA (normally used for producing proteins). Cells at this stage are called reticulocytes, and circulate in the bloodstream for about a day, during which they destroy their remaining RNA and become fully mature red blood cells (erythrocytes). In healthy adults, 1 to 5 in every 200 circulating red blood cells are reticulocytes.

Monocytes usually circulate for around 3 days after exiting the bone marrow before becoming a macrophage or dendritic cell. Lymphocytes, on the other hand, must first migrate to the thymus, where they undergo rigorous “vetting” to ensure that they will not react to the body’s own antigens.

What Happens In Diseases of the Bone Marrow?

There are many types of bone marrow disease, and they usually involve the overproduction or underproduction of blood cells. Overproduction can be caused by cancers of the bone marrow (leukemia) and genetic conditions. Underproduction can be caused by damage to the bone marrow originating from toxic substances, radiation, pathogenic infection and autoimmune attack. Certain medications, pregnancy and genetic conditions can also cause blood cells to be underproduced.

When too many red blood cells and platelets are produced, this can lead to an increased risk of blood clot formation, blocking blood vessels and causing stroke and heart attack. Overproduction of white cells can weaken the immune system when one type of immune cells is overproduced to the detriment of other types. Underproduction of red blood cells can lead to anaemia, while a lack of platelets causes increased bruising and bleeding due to defective blood clotting.

What About Transplants?

A human cannot survive without a functioning bone marrow. Fortunately, blood stem cells have the ability to divide, migrate, and repopulate bone marrow in which stem cells have been destroyed. It is therefore possible to transplant the bone marrow from one individual to another.

There are 2 types of bone marrow transplants: allogeneic and autologous. Both types take healthy bone marrow cells and use them to replace the patient’s diseased or damaged cells.

In an allogeneic bone marrow transplant, a healthy person’s bone marrow cells are removed and given to the patient. Most donated bone marrow comes from a sibling, but it can also come from a more distant relative or an unrelated donor.

In an autologous bone marrow transplant, some of the patient’s own healthy bone marrow cells are harvested. These cells can then be given back to the patient as a transplant after their existing bone marrow has been destroyed. This does not by itself fix genetic conditions, as any disease-causing mutation will be carried by all of the patient’s stem cells. However, thanks to gene editing technology, it is becoming possible to correct genetic mutations in harvested stem cells prior to transplantation, effectively curing the genetic disease.