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What is Myelofibrosis? 

Myelofibrosis (MF) is caused by an abnormality in the cells that make platelets. It results in bone marrow being replaced by scar (fibrous) tissue.

Myelofibrosis is a disorder of the bone marrow. It occurs when the marrow – the soft, fatty tissue inside your bones that produces stem cells – is replaced by fibrous (or scar) tissue. Scarring of the bone marrow means the marrow is not able to make enough blood cells. The liver and spleen try to compensate by producing their own blood cells, but this can cause them to swell. MF can also lead to anemia and bleeding problems, as well as a higher risk of infections.


Our expert physician
Professor Claire Harrison explains MF

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Learn more about the role of bone marrow and stem cells



A cross section of bone (marrow)

Bone marrow is the soft, spongy tissue that fills the inner cavities of the bone. It's where red blood cells, white blood cells and platelets are formed. It also contains fluid, blood vessels and fat.


The bone marrow is either red or yellow. When we are born, we only have red marrow, which is responsible for creating new blood cells. As we get older, some of the red marrow is replaced by yellow marrow, which is largely fat. A healthy adult has both red and yellow marrow.

MF can occur on its own as primary myelofibrosis – and usually develops slowly in people over the age of 50 – or as a progression of other bone marrow disorders such as polycythaemia vera (PV) and essential thrombocythaemia (ET), resulting in post-polycythaemia vera MF and post-essential thrombocythaemia MF.



How rare is myelofibrosis?

The annual incidence of MF is between 0.4 and 1.5 people out of every 100,000. MF can be diagnosed at any age, however is most commonly diagnosed in patients between 60-70 years. MF affects men and women in relatively equal numbers. Approximately one sixth of patients with MF had previously been diagnosed with ET or PV.