What is bone marrow?

The essential roles of our skeletons' spongy substance

(Image source: Chad McNeeley)

Bone marrow, also known as myeloid tissue, is a soft jelly that is found in the bone cavities. It consists of immature cells called stem cells, which develop into red blood cells. In humans, red bone marrow creates all the major blood cells in the body. There are three blood cell types; the red carries oxygen through the body while white cells ingest bacteria and fight infection. The third type of blood cell, the platelets, aid clotting after injury. It is clear, therefore, that bone marrow has an important role in human development. While red bone marrow cleans up the body, aiding the liver and spleen to destroy old cells, the yellow bone marrow helps to store fat and provide energy.

Disorders of bone marrow can threaten life. Sometimes the body is depleted of iron and is unable to create normal red blood cells, or the overproduction of one type of cell can cause serious health risks. Malfunction prevents the natural development of blood cells and instigates the onset of disease, such as leukaemia, lymphoma or aplastic anaemia. 

When a person needs a bone marrow transplant, it means that a doctor can take haematopoietic stems cells from one individual and transfer them to the body of a recipient. If the donor and recipient are compatible, the cells will travel to the area of the diseased bone marrow and instigate new cell production.

Bone marrow transplantation is successfully practised around the world. Harvesting bone marrow is a relatively simple operation that is usually performed under general anaesthetic. Here the stem cells are harvested from the red marrow in the bone. The operation is conducted using a long needle inserted into the soft centre of the bone, the marrow is drawn through a hollow needle and removed from the body. Often, one litre of marrow is extracted. It is placed in bags, processed and stored at low temperatures.

Autologous bone marrow transplants are performed when stem cells are harvested and then returned to the body after the patient has been treated for their cancer. Allogeneic bone marrow transplants rely on a donor who has the same bone marrow type as the patient.

Bone marrow close up

Cells that make up bone marrow are classed as fibroblasts, macrophages, adipocytes, osteoblasts, osteoclasts and endothelial cells. Each cell has a particular function and helps to maintain the general health of the body. Sickness occurs when a cell, or group of cells, malfunction, and the patient requires treatment through a bone marrow transplantation.

(Image credit: Alexander Klepnev)

1. Fibroblasts

These play a critical role in the healing of wounds on the body.

2. Macrophages

From the Greek, meaning ‘big eaters’ they are the soldiers of the body, working against disease.

3. Adipocytes

Adipocytes store energy in the body in the form of fat.

4. Osteoblasts

Osteoblasts are responsible for the formation and growth of bones.

5. Osteoclasts

Osteoclasts are responsible for breaking up organic bone in the body.

This article was originally published in How It Works issue 14

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