Although bone marrow fibrosis is a lethal condition, its underlying mechanism is not fully understood. This study aimed to investigate the pathogenesis of fibrosis in the bone marrow through histologic examination of mast cell infiltration and the expression of fibrosis-associated cytokines. We analyzed 22 bone marrows with fibrosis (8 primary myelofibrosis [PMF], 5 post-essential thrombocythemia [ET], myelofibrosis, and 9 myelodysplastic syndrome [MDS] with bone marrow fibrosis [BMF]). Immunohistochemical and immunofluorescence stainings were performed using anti-mast cell tryptase, interleukin (IL) 13, transforming growth factor β (TGF-β), CD34, and CD42b antibodies. The number of mast cells in bone marrows with fibrosis was significantly higher than that in controls (P
