Intravascular Injection of Murine Mesenchymal Stem Cells in Non-Injured Neonatal Mice Results in Bone Forming Nodules within Pulmonary Vessels.

Murine mesenchymal stem cells (mMSC) are capable of differentiation into multiple cell types in vitro, and as a result have been proposed as ideal candidates for the development of cellular based therapy for tissue repair. The distribution, engraftment, and differentiation properties of these cells in vivo have yet to be conclusively defined. Reports of mMSC engraftment and differentiation are confounded by issues of cell population purity, questions of fusion, a reliance on pathologic injury models to demonstrate significant engraftment levels, and immunofluorescent based cell tracking modalities which are prone to artifact and rely on subjective interpretation. In this study, we utilize a non-injured neonatal mouse model to demonstrate the fate of culture expanded, purified mMSC derived from the bone marrow of adult GFP transgenic mice. Following intravascular injection into Swiss Webster neonatal mice on the first day of life, mMSC were tracked with fluorescent stereomicroscopy and corresponding tissue specimens were collected and processed for histology and immunoperoxidase staining for GFP. mMSC were found to concentrate within the lungs with few, isolated cells visualized outside the lungs. Histological examination confirmed the presence of GFP expressing cells within the pulmonary vasculature but occurring as aggregates of bone. These nodules were found as early as 9 days after injection and persisted to 42 days after injection. These nodules were located within the confines of the vessel wall and were surrounded by an inflammatory infiltrate. There was no evidence of tissue invasion nor did these cells demonstrate malignant features. These bony aggregrates were also demonstrated after injection into C57Bl/6 congenic controls. Our findings indicate that mMSC are filtered by the pulmonary vasculature following intravenous injection thereby limiting their capacity to reach other tissues. The large adherent phenotype of these cells makes them prone to vessel lodgment, and this lodgment results is aberrant osteogenesis. These findings have major implications for intravascular stem cell based therapies for tissue repair.