The hypothesis of mitochondrial transfer between eukaryotic animal cells is intriguing, although its route of action and physiological role is unknown. Our goal was to examine intercellular connections among several types of stem cells and to observe whether intact functional mitochondria may travel via these connections. Time-lapse laser scanning confocal microscopy has shown that human amnion-derived stem cells as well as bone marrow derived mouse and human mesenchymal stem cells form cell-to-cell connections via a tubular membrane network. The maximal length of these micrometer-thick tubes is around 180 ?m. Interestingly, freshly isolated amniotic epithelial stem cells did not form these connections, only after several passages when the morphology of the cells is significantly altered. Large area cell-cell contacts can be retained as long thin membrane bridges after the cells depart and de novo tube formation is also observed. Using MitoTracker red staining we observed that intact mitochondria are moving in these tubes by 20 – 60 nm/s velocity, suggesting that mitochondria can leave one cell via the membrane tubes and can enter into another cell. These results suggest that specific types of stem cells form comprehensive tubular networks among each other. One physiological role of these networks may be that mitochondria can migrate from one cell to the other, which may be a novel way of communication among stem cells.