During the murine gestation period cut the embryos off of the umbilical cord but leave one or two so that the placenta remains active. Then attach the freed umbilical cords to an old, genetically identical animal’s blood circulation. Keep the mice fixated together throughout the gestation period. This would effectively be dialysis through the placenta - the ultimate stem cell and young blood therapy.
Does the old animal get rejuvenated?
Repeat the experiment but throughout it include aggressive therapies aimed at eliminating problematic cells (senolytics, chemotherapy, etc). Does this change the outcome as compared to the previous experiment?
Where this could lead eventually
People of the future could bioengineer a rejuvenation organ (permanent mini placenta) which on-demand and indefinitely provides stem cells and any other factors necessary for rejuvenation. With such an organ inside the body, people could focus further on therapies at killing/removing any cells suspected of being damaged.
Alternatively, there could be a device (like a small bioreactor of trophoblast cells) to which humans could periodically connect for rejuvenation dialysis.