QD labeled MSC in cardiac myocyte co-culture at 7 days. Images show optical sections acquired as a confocal Z stack with 1-μm spacing. Image a shows QD labeled MSC above cardiac myocytes. As images advance into the cell culture (a – e, towards coverslip) the QD labeled MSC is shown adjacent to and surrounded by cardiac myocytes. All cells were labeled with the cytosolic fluoroprobe calcein AM. QDs are preferentially localized in the MSC. No QDs were found to be localized in the cardiac myocytes. All images were acquired with an oil immersion 40× objective. Scale bar 20 μm.