The uterine endometrium changes periodically from proliferating to shedding every month, and a sufficient energy supply is necessary to maintain its active physiological state. Abundant blood support from the uterus also shows that the endometrium is a tissue with very active oxygen metabolism . When the endometrium is affected by mechanical damage or inflammation, it is in a state of ischemia and hypoxia . Mitochondria are the most important organelles for providing energy to maintain normal cell function . Under hypoxic conditions, the mitochondrial electron transport chain is disordered because there are not enough oxygen molecules to act as the final acceptor of electrons, and a large amount of reactive oxygen species (ROS) is produced through the electron transport chain. Excessive ROS destroys the balance between ROS and antioxidant levels and finally leads to disorder of endometrial cell proliferation [19, 20]. Our study also confirmed that when endometrial stromal cells are in a state of hypoxia, ATP production in mitochondria is significantly reduced and the proliferation of hypoxic cells is impaired. Lysosomal hydrolase release and lysosomal dysfunction are found in endometrial stromal cells, which may lead to mitochondrial dysfunction . Our previous studies on mitochondrial DNA also found that the polymorphism of genes that transcribe and translate proteins related to oxidative phosphorylation in mitochondrial DNA can affect the repair and proliferation of the endometrium, which indirectly confirmed that the change of mitochondrial function is related to the repair and proliferation of the endometrium .
Mitochondria are organelles capable of active transcellular transfer. When BMSCs were injected into the airways of mice with acute lung injury, mitochondria of BMSCs were found in the lung epithelial cells of the mice, confirming the presence of mitochondrial transfer. Moreover, mitochondria that transferred into local alveolar cells increased the ATP of local and adjacent alveolar cells and the secretion of surface-active substances of type II alveolar cells, showing an obvious repair effect on injured cells [10, 23]. We found in our study that when hUCB-MSCs and endometrial stromal cells pretreated with hypoxia were cocultured in vitro, the red fluorescently labeled mitochondria of hUCB-MSCs were transferred into endometrial stromal cells, ATP production was increased, and cell proliferation was partially restored. When mitochondrial transfer is inhibited, ATP production and cell proliferation are also inhibited. These findings indicate that there is mitochondrial transfer between MSCs and hypoxic endometrial stromal cells, and mitochondrial transfer is a way of functional recovery of hypoxic endometrial stromal cells. In addition to alveolar cells and endometrial cells, the phenomenon of mitochondrial transfer also exists in nerve cells , vascular smooth muscle cells , cardiomyocytes  and various malignant tumor cells , suggesting that the transcellular transfer of mitochondria is a biological phenomenon of cells under stress. Studies have even found that human mitochondria can be found in the alveolar epithelial cells of mice after infusing human bone marrow mesenchymal stem cells into acute lung injury model mice, indicating that mitochondrial transfer is not limited by cell types and populations . Supplementing stem cells and increasing mitochondrial transfer between cells may be a way to improve the prognosis of endometrial injury.
Similar to other cell-to-cell signaling pathways, mitochondrial transcellular transfer also takes place through connections between cells . Tunneling nanotubes (TNTs) are a type of long membrane structure 100–800 nm in width and 100 µm in length. Based on F-actin as their framework, TNTs are wrapped by a phospholipid bilayer extending from the cell membrane, connecting the cytoplasm of two cells. They exist extensively in a variety of physiological and pathological cells, such as kidney cells, astrocytes, and myocardial cells. CBX can block cells from forming tunneling nanotubes (TNTs) to connect with other cells. In this study, CBX prevented hUCB-MSCs from forming TNT connections with endothelial stromal cells. Intercellular mitochondrial transfer was also significantly affected, ATP production in hypoxic endometrial stromal cells was reduced, and cell proliferation was inhibited. This indirectly indicates that hUCB-MSCs can form TNT connections with hypoxic endometrial stromal cells. This linkage is a major mode of mitochondrial transfer between hUCB-MSCs and hypoxic endometrial stromal cells. In addition to endometrial stromal cells, TNT connections are the main mode of intercellular communication in the mitochondrial transfer between BMSCs and damaged alveolar epithelial cells or hypoxically injured cardiomyocytes [31, 32]. Furthermore, in our study, we blocked the formation of TNT junctions in hUCB-MSCs rather than hypoxic endometrial stromal cells. Mitochondrial transfer was significantly affected, indicating that stem cells form intercellular connections with injured endometrial stromal cells and actively achieve mitochondrial transfer rather than passively.