Bone Marrow and Blood Formation
Bone marrow is the spongy tissue that fills the cavities inside your bones. Most of your blood cells are made in your bone marrow. The process by which blood cells are made is called haemopoiesis.
As an infant, haemopoiesis takes place at the centre of all bones. In later life, it is limited to the hips, ribs and breastbone (sternum). Some of you may have had a bone marrow biopsy taken from the bone at the back of your hip (the iliac crest).
You might like to think of the bone marrow as the blood cell factory. The main workers at the factory are the blood stem cells. They are relatively small in number but are able, when stimulated, to reproduce vital numbers of red cells, white cells and platelets. All blood cells need to be replaced because they have limited life spans.
There are two main families of stem cells, which develop into various types of blood cells.
Myeloid (‘my-loid’) stem cells develop into red cells, white cells (neutrophils, eosinophils, basophils and monocytes) and platelets.
Lymphoid (‘lim-foid’) stem cells develop into two other types of white cells called T-cells and B-cells.
Growth factors and cytokines
All normal blood cells have a limited survival in circulation and need to be replaced on a continual basis. This means that the bone marrow remains a very active tissue throughout your life. Natural chemicals in your blood called growth factors or cytokines control the process of blood cell formation.
Different growth factors stimulate the blood stem cells in the bone marrow to produce 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 erythropoeitin (EPO) stimulates the production of red cells.
Blood consists of blood cells and plasma. Plasma is the straw coloured fluid part of the blood that blood cells use to travel around your body. The blood cells consist of red cells, white cells and platelets.