Labelling and tracking of human mesenchymal stromal cells in preclinical studies and large animal models of degenerative diseases.

Success of stem cell therapies were reported in different medical disciplines, including haematology, rheumatology, orthopaedic surgery, traumatology, and others. Currently, more than 4000 clinical trials using stem cells have been completed or are underway, among which 378 investigated or are at present investigating mesenchymal stromal cells (MSCs). The majority of clinical trials using stem- or progenitor- cells, including hematopoietic stem cells and MSCs, target the immune system. However, therapies based on MSCs are increasingly implemented to treat symptoms in which failure of the resident stem cells in situ, or malfunction of tissues or structures are not associated with immune cells or inflammation, but instead are associated with mechanical or metabolic stress, ageing, developmental or acquired malformations, and other causes. To proceed further in the development of stem cell therapies as a safe and effective treatment for surgical and other medical specialities, the behaviour of MSCs implanted in preclinical models and their impact on the site of application need to be explored in detail. Depending on the pre-clinical model employed, tracking of labelled stem cells in live animals makes an enormous difference for exploration of the mechanisms and kinetics involved in MSC-mediated tissue regeneration. Here we review (pre-)clinically applicable key methods to label human MSCs for short and long-term observations in small and large animal models.