The bone marrow and blood formation

Bone marrow is spongy tissue in the middle of certain bones. Most blood cells are made in your bone marrow. This process is called haemopoiesis.

In children, haemopoiesis takes place in the long bones, like the thighbone (femur). In adults, it’s mostly in the spine (vertebrae) and hips, ribs, skull and breastbone (sternum). You may have a bone marrow biopsy taken at the back of your hip (the iliac crest).

How blood is produced

Think of blood production like a family tree. At the top of the tree are the blood stem cells (or hematopoietic stem cells), which are the youngest (most immature) blood-forming cells. They can make copies of themselves. They also make new cells that are closer to being blood cells, called progenitor cells.

There are two types of progenitor cells that split the family tree: lymphoid cells and myeloid cells. These cells then develop into various types of blood cells:

Myeloid stem cells develop into red cells and some white cells (neutrophils, eosinophils, basophils and monocytes) and platelets. Immature myeloid stem cells are called myeloblasts (or just blast cells).

Lymphoid stem cells develop into T-cells and B-cells. Immature lymphoid stem cells are called lymphoblasts (or just blast cells).

Platelets are made from very large bone marrow cells called megakaryocytes. These are formed in the myeloid part of the tree. When megakaryocytes break apart, they form more than 1000 platelets each.

Finally, at the bottom of the family tree are the mature red blood cells, white blood cells, and platelets.

Growth factors

All normal blood cells live a short time: red blood cells 80–100 days, neutrophils 8–14 days, and platelets 4–5 days. They then die off and are replaced by new cells from the bone marrow. This means that your bone marrow remains very busy throughout your life.

Chemicals in your blood called growth factors control blood cell formation. Different growth factors make the blood stem cells in the bone marrow become different types of blood cells.

These days some growth factors can be made in the laboratory (synthesised) and are available for use in people with blood disorders. For example, granulocyte-colony stimulating factor (G-CSF) stimulates the production of white cells called neutrophils while erythropoietin (EPO) stimulates the production of red blood cells.