I am an undergraduate student who recently appeared for Hannover Medical School Phd Entrance examination.
They asked many questions to test analytical abilities one of which was" How will you prove in vivo that a cell is stem cell" ? Though I have very less knowledge in this field I gave the following answer on which many of you may laugh.
I answered that , Since stem cells have characteristic property of multidifferentiating, it can be exploited to prove its stem cell-ness. Two factors- one which induces tumor for example, and other which makes an animal immune to a X pathogen, can both be injected into yolk sac or bone marrow. If the tumor developed and animal is protected against infection upon exposure to that X pathogen, we can say vaguely that it has stem cell properties as it was able to give rise to two types of different cells. Its a very general answer and do not include any technical know-how.. If any of you can make any suggestions I will be happy.