Mesenchymal stem cells and progenitor cells in connective tissue engineering and regenerative medicine: is there a future for transplantation?

Transplantation surgery suffers from a shortage of donor organs worldwide. Cell injection and tissue engineering (TE), thus emerge as alternative therapy options. The purpose of this article is to review the progress of TE technology, focusing on mesenchymal stem cells (MSC) as a cell source for artificial functional tissue. MSC from many different sources can be minimally invasively harvested: peripheral blood, fat tissue, bone marrow, amniotic fluid, cord blood. In comparison to embryonic stem cells (ESC), there are no ethical concerns; MSC can be extracted from autologous or allogenic tissue and cause an immune modulatory effect by suppressing the graft-versus-host reaction (GvHD). Furthermore, MSC do not develop into teratomas when transplanted, a consequence observed with ESC and iPS cells. MSC as multipotent cells are capable of differentiating into mesodermal and non-mesodermal lineages. However, further studies must be performed to elucidate the differentiation capacity of MSC from different sources, and to understand the involved pathways and processes. Already, MSC have been successfully applied in clinical trials, e.g., to heal large bone defects, cartilage lesions, spinal cord injuries, cardiovascular diseases, hematological pathologies, osteogenesis imperfecta, and GvHD. A detailed understanding of the behavior and homing of MSC is desirable to enlarge the clinical application spectrum of MSC towards the in vitro generation of functional tissue for implantation, for example, resilient cartilage, contractile myocardial replacement tissue, and bioartificial heart valves.