PTH increases mast cell number and secretion of mast cell–derived preformed effectors in rats.(38,39) To date, studies have emphasized the role of mast cell–derived histamine in osteoclastogenesis and bone resorption.(40–44) The role of mast cells in osteoblast differentiation and HPT-induced bone marrow fibrosis has received less attention. This evidence concerns the gene PTH and primary myelofibrosis.