['For approximately 5% of autologous transplant recipients and a higher proportion of allogeneic transplant recipients, low level and delayed platelet engraftment is an ongoing problem. Mesenchymal stem cells (MSC), which can be derived from bone marrow as well as other organs, are capable of differentiation into multiple cell types and also support hematopoiesis in vitro. Because cotransplantation of marrow-derived stromal cells has been shown to enhance engraftment of human hematopoietic stem cells, we hypothesized that cotransplantation of MSC could enhance platelet and myeloid cell development.', 'We tested this hypothesis by transplantation of CD34-selected mobilized human peripheral blood stem cells (PBSC) into sublethally irradiated NOD/SCID mice with or without culture-expanded human MSC and evaluated human myeloid, lymphoid, and megakaryocytic engraftment with flow cytometry and in vitro cultures.', 'We find that MSC cotransplantation enhances human cell engraftment when a limiting dose (<1 x 10(6)) of CD34 cells is administered. This enhancement is characterized by a shift in the differentiation of human cells from predominantly B lymphocytes to predominantly CD13(+), CD14(+), and CD33(+) myeloid cells with a corresponding increase in myeloid CFU in the marrow. Megakaryocytopoiesis is enhanced by MSC cotransplantation as assessed by an increase in both marrow CFU-MK and circulating human platelets. In contrast, MSC do not affect the percentage of human bone marrow cells that expresses CD34(+).'] Does cotransplantation of human mesenchymal stem cells enhance human myelopoiesis and megakaryocytopoiesis in NOD/SCID mice?