Embryonic stem cells are the cells which specialize into all the different parts of a body in early development. In the lab these cells are grown after in vitro fertilization (IVF), the process where an egg is fertilized outside of a human body with the full informed consent of the donor. These cells can proliferate for a year in the lab becoming millions of cells and can differentiate into any cell in the body - they are extremely versatile. Adult stem cells on the other hand are believed to be more specialized, differentiating into cells which are more specific to where they were taken from the body. However, there have been reported incidents of adult stem cells specializing into other cell types very different from their location, such as a stem cell which would usually differentiate into a blood cell instead differentiating into a muscle tissue cell. This process is called trans-differentiation. While this has been observed in other animals it is very rare and it is still debated as to whether it happens in humans at all. The advantage of using adult stem cells in medicine is that they can be taken directly from the patient’s body, specialized and then returned. The patient's immune system is less likely to reject these cells because they are from the patient’s body originally.