The aim of this study is to demonstrate that, in patients with myelofibrosis (PMF), a chronic neoplastic disease of the bone marrow, some of the cells that form the bone marrow microenvironment (mesenchymal stromal cells, MSCs) can send wrong messages to the CD34+ hematopoietic progenitor cells (HPCs). CD34+ HPCs in normal conditions reside in the bone marrow until complete maturation; in PMF they leave the bone marrow when they are still progenitor cells and frequently they express the V617F mutation on the JAK2 molecule. In previously published papers, the investigators demonstrated that the MSCs in the bone marrow of patients with PMF are different from those in healthy donors. In this study the investigators want to verify how this difference affects the maturation of CD34+ HPCs of healthy donors and of SET2 cells, a commercially available cell line characterized by the V617F mutation of the JAK2 molecule. In particular, the investigators will verify the effect of MSCs on signals that induce the activation of CD34+ cells, their survival or death, the production of harmful oxidative reagents and their clonal capacity. To obtain these data, the investigators will isolate MSCs from bone marrow blood samples of patients with PMF undergoing bone marrow biopsy for clinical reasons and from healthy subjects, donors of HPCs for transplant. Following a long culture MSCs will be stimulated to release small vescicles containing part of their nuclear and cytoplasmatic molecules. These vescicles will be cultured with CD34+ cells or SET2 cells and their effects will be evaluated. It has been shown that these small vescicles act exactly like MSCs, but they are easier to be used, and could become a very useful acellular drug in the near future.