| It has been well demonstrated that the bone marrow contains a population of CD34+ hematopoietic stem cells (HSCs). However, the bone marrow also contains a second cell type, which meets most of the criteria for stem cells of non-hematopoietic tissues. Because of their multipotential differentiation capacity, these cells are currently known as mesenchymal stem cells or marrow stromal cells (MSCs). We have determined that cultures of human MSCs from different donors are heterogeneous showing varying growth and differentiation potentials. As samples were expanded in culture and approached senescence, they retained their ability to differentiate into osteoblasts. However, the cells failed to differentiate into adipocytes. The loss of multipotentiality after expansion presents a problem for MSC experimentation and their use as a therapeutic vehicle.; Three specific aims have been addressed in the following dissertation. The first specific aim was to develop a protocol for the expansion of MSCs in culture with an emphasis on distinguishing the most primitive cell type within the heterogeneous culture. The second aim was to identify the surface protein phenotype of the primitive MSC. Finally, the third aim was to determine the growth factors involved in MSC expansion in culture.; In this dissertation we have determined the conditions by which MSCs can be expanded 600–2000 fold in culture while maintaining their primitive attributes. This is an attractive characteristic for the use of MSCs as a therapeutic delivery vehicle. We have also identified a unique subset of human MSCs that are small, proliferate rapidly, undergo cyclical renewal, and are precursors of more mature cells in the primary culture. We have therefore termed these cells bone marrow derived, rapidly self-renewing stem (RS) cells. We have identified several protein epitopes (FLK-1, TRK, annexin II, transferrin receptor) that distinguish RS cells from mature cells and can be used to purify the RS cells from cultures of MSCs. We have also identified an autocrine/paracrine regulatory mechanism that regulates RS cell expansion and self-renewal in culture. These studies will ultimately allow for the exploitation of RS cells as a therapeutic delivery vehicle for cell and gene therapy. |
