The self-renewal procedures of mesenchymal stem cells in the blood

Background Although mesenchymal stem cells (MSCs) are most commonly used in cell therapy and stem cell research, the mechanism and the locations of their self-renewal are still unknown. Method Mouse blood was collected, and examined under microscopy. The results were compared with the data of human umbilical cord blood (hUCB) collected 10 years ago. Results We found that the procedure of self-renewal for the mesenchymal stem cells in mouse blood and hUCB needs at least 5 steps. First, tube-shaped stem cell niches release long segmented materials composed of sand-like particles and semitransparent granules. Second, the sand-like particles and semitransparent granules separate from the segmented materials. Third, each of the individual semitransparent granules releases groups of fusiform-shaped structures that do not stain to H&E. The sizes of the fusiform-shaped structures range from 1 to 100 μm in length in mouse blood, but can be 200 μm in hUCB. Fourth, the large-sized fusiform structures can directly transform into lineage-restricted cellular structures; the medium-sized fusiform structures fuse or engulf each other to form cellular structures. The cellular structures further acquire membranes from the adjacent nucleated cells. Fifth, the nucleolus appears in the new cellular structures before the nucleus. During all the procedures, the adjacent nucleated mesenchymal cells are must needed. Thus, these newly formed cellular structures will further differentiate into nucleated mesenchymal stem cells. Conclusion Our findings again provide new evidence that, in physiological conditions, mesenchymal stem cell self-renewal needs several steps to complete, which, however, does not occur by mitotic division. The tube-shaped structures are the niches of the stem cells.