Musculoskeletal diseases are the most commonly reported health conditions in the United States.1 These diseases include various forms of arthritis, congenital deformities and anomalies, fractures, and pain associated with the back, neck, or intervertebral disks.1 Millions of surgeries are performed every year to correct musculoskeletal diseases, yet a high percentage fail to obtain a satisfactory outcome. 2–5 For example, only 64% of patients treated surgically for lumbar spinal stenosis reported good-to-excellent outcomes.4 Treatments and lost wages due to musculoskeletal diseases are a significant burden on society, representing 7.7% gross domestic product (~$849,000,000,000) for 2002 to 2004.1 Therefore, new therapies that reduce the cost and morbidity of musculoskeletal diseases are in great demand. MSCs have been the focus of widespread attention in recent years, and are being studied in over one hundred clinical trials world-wide.6 In addition to their obvious potential for treatment of musculoskeletal diseases, MSCs have demonstrated promise as a cell therapy in many pre-clinical and early-stage clinical trials for a range of diseases, including diabetes, cardiac disease, bone marrow transplant-associated GVHD, and osteogenesis imperfecta.6,7 Although proven to be safe, efficacy in late stage clinical trials has not met expectations.8,9 This has led some to develop strategies that can enhance the potency of MSCs before infusion, including activation or transfection of cells before infusion, and cell surface engineering.10,11 A critical challenge in the development of these enhancement strategies is quantification of in vivo MSC homing and therapeutic efficiency. To address this challenge, we applied in vivo confocal and multi-photon microscopy, a powerful singe-cell detection and evaluation technique. MSC use in pre-clinical and clinical models began with their discovery in the 1960’s during bone marrow transplant experiments that led to the hypothesis that a cell type existed within the bone marrow that could differentiate into osteoblasts, aid in the development of sinusoidal structures, and support hematopoiesis in ectopic sites.12 These skeletal stem cells were first purified from bone marrow based on adherence to tissue culture plastic and were re-named MSCs for their ability to differentiate into adipocytes, chondrocytes, and osteoblasts.13,14 MSCs have been well characterized in vitro, and are defined as being tissue culture plastic adherent; positive for CD105, CD73, and CD90 antigens; negative for HLA-DR, and CD45, CD34, CD14, CD19 antigens; and able to differentiate into osteoblasts, adipocytes, and chondrocytes.15 Recently, it has been suggested that MSCs are perivascular cells in vivo. This has raised many interesting questions about how pericytes and adventitial cells respond to acute bone injury and if perivascular cells in non-bone tissues are also MSCs.16–18 Our research describes the ability of in vivo confocal and multi-photon microscopy to quantify the behavior of exogenous engineered MSCs and evaluate endogenous perivascular cell response to bone injury. Our results contribute to our understanding of how exogenous MSCs interact with diseased tissue and how endogenous perivascular cells respond to musculoskeletal injury. These results may help improve current cell therapies and lead to the development of novel therapeutics.
