Last edited 12/2021 and last reviewed 01/2022

Myelofibrosis (also known as primary myelofibrosis, agnogenic myeloid metaplasia, chronic idiopathic myelofibrosis, myelofibrosis with myeloid metaplasia, and idiopathic myelofibrosis) is a chronic myeloproliferative disorder characterised by (1,2):

  • varying degrees of fibrosis of the bone marrow
  • extramedullary haemopoiesis with splenomegaly
  • anaemia with leukoerythroblastosis with teardrop poikilocytosis in peripheral blood (3)

The term “myelofibrosis” is being used to describe any increase in bone marrow stromal fibres (without considering the type of fibre or the associated disease) (4)

Marrow fibrosis is thought to occur as a result of increased secretion of platelet derived growth factor.    

In myelofibrosis patients the disease initiating mutation is unknown. In a majority, a somatic Janus kinase 2 (JAK2) mutations (JAK2V617F) and in a minority MPL, LNK mutation is present (5)

  • the discovery of mutations in JAK2, CALR, and MPL have uncovered activated JAK-STAT signaling as a primary driver of MF, supporting a rationale for JAK inhibition (6)

Loss of marrow capacity results in extramedullary haematopoiesis in the liver, spleen and lymph nodes.