Myelofibrosis is a progressive cancer in which the bone marrow, or the spongy tissue inside bones that helps blood cells grow, is gradually replaced by fibrous, scar-like tissue. This scarring process slowly reduces the marrow’s ability to make blood cells. As a result, the body cannot make enough of the cells that fight infection (white blood cells), carry oxygen (red blood cells), and help clot blood (platelets). Not having enough of these cells can result in anemia (leaving patients feeling weak or tired), infections and bleeding. As the disease progresses, the body attempts to make blood cells outside the bone marrow, notably in organs such as the spleen and liver, which slowly become enlarged.
Myelofibrosis is caused by genetic changes (mutations) that arise by chance in bone marrow stem cells (the cells inside the bone marrow that make blood cells). These changes are almost never passed on by previous or to future generations.