['The biodistribution of human MSCs after systemic delivery is incompletely understood. We investigated the changes in cell size and cell surface markers of human MSCs after intravenous (IV) injection in immune competent mice.', 'Male human MSCs were labeled with fluorescent vital dye PKH67 and tracked after IV administration in C57/BL6 mice. MSCs were tracked in blood and different murine tissues by human SRY gene quantitative polymerase chain reaction (qPCR) analysis, flow cytometry and fluorescence microscopy. Calibrated microbeads were used to track the size of transplanted MSCs.', 'The majority of injected MSCs were detected by qPCR in the lungs 5\u2009min after transplantation, whereas <0.1% were detected in other tissues over 24\u2009h. Flow cytometric and fluorescence microscopic analysis indicated that MSCs continuously decreased in size after transplantation and underwent fragmentation. The majority of PKH'] Do human mesenchymal stromal cells undergo apoptosis and fragmentation after intravenous application in immune-competent mice?